There are two main methods of obtaining compressed air, a SCUBA tank and a Hand Pump. SCUBA stands for Self Contained Underwater Breathing Apparatus, so its intended purpose should be obvious. The mass of kinetic energy inside is ideal for use in pcp's, but this energy also needs to be treated with respect.  

The following abbreviations will be used from now on.
1> SCUBA tanks will be referred to as 'tank'.
2> The cylinder (fixed or detachable) fitted to the weapon will be referred to as 'cylinder'. 
 
The use of the hand pump is covered in the Hand Pumps subject on this page, so apart from the safety issues that apply to cylinders, this section will concentrate on tanks.

Tanks come in various sizes and pressure rating. Most tanks are rated at 200 to 232bar (2900 to 3365psi) or 300bar (4351psi). Which type to choose is a decision that only the individual can make. Size, weight and cost are all factors in this choice, but the number of times a cylinder can be refilled is also a major consideration and is directly related to the size and pressure rating of the tank.
From some of the post and telephone enquiries received, there is obviously confusion about the relationship between tank pressure, cylinder pressure and what happens when you connect the two together. To try and illustrate this process without cutting too fine a point, a theoretical example is detailed below.
Controlling factors in this example:
1> SCUBA tank size = 3litres
2> SCUBA tank pressure = 300bar
3> Cylinder size = 211cc
4> Cylinder filling pressure = 190bar
5> Cylinder to be refilled when at 100bar, the lower operating pressure of the gun.
As a matter of interest if the cylinder is empty when you fill it to190bar there will only be 280bar left in the tank, but for this exercise lets assume the cylinder has 100bar left in it (the pressure at which the cylinder needs refilling after use). In this case the tank will be left with 287bar.
In theory the tank will give 11 fills to 190bar, but the 12th fill will only put 188bar into the cylinder because the combined volume of the tank and cylinder are too large for the remaining air in the tank to give 190bar in both. 
Now, every time the cylinder is filled, the result is that the tank and cylinder equalize in pressure, so the 13th fill ends up at 183bar, 7bar less than what will give the full number of shots. At the 22nd fill the tank and cylinder are at 145bar, in other words you only get half as many shots than when filled to 190bar.  
Unless the tank is refilled to 300bar at this point, the reductions will continue. Once the equalized pressure is down to 100bar, although you may still be able to keep firing, the power will be dropping away dramatically.

At the start of the last paragraph the phrase 'In theory' was used and that is because the calculations assume 100% efficient usage of the decanted air, which will never happen. Just venting and disconnecting the hose will waste air and this will effect the true number of fills available from the tank.
If we take this further and use the same example but use a 232bar tank instead of 300bar, the results are quite different.
Now the tank will only give 5 fills to 190bar, the 6th will equalize at 188bar.  
These factors must be taken into account when considering the best tank size and pressure for your needs.

Safety Issues:
If simple safety rules are applied to the filling, use and storage of tanks and cylinders the dangers are miniscule, but ignore them and there is potential for serious personal injury. The danger actually comes from not knowing or applying the safety rules not the tanks and cylinders themselves.  
Even a relatively small 3ltr tank filled to 300bar has an enormous amount of energy stored in it and if this energy is released in an uncontrolled manner you will not want to be near it. The same can be said of a 211cc cylinder, only to a lesser degree.

SCUBA:
Filling: The filling of tanks should always be safe if taken to a proper diving shop. The diving industry is well regulated and fully aware of the dangers involved. If you take a damaged or uncertified tank for refill you will almost certainly be turned away, worth remembering if you are considering buying a secondhand tank. If you are able to get tanks filled outside of the normal diving services, beware! It may be cheap and/or convenient but you could be putting yourself and others at risk. NEVER be tempted to fill a tank over its design pressure, this is highly dangerous, illegal and will drastically reduce the working life of your tank.  
Use and storage: The most important safety rule is to prevent damage to the valve and tank walls. Any dent or deep scratch in the tank wall is a potential stress fracture point. Secure the tank so that it can't fall over. If it does, and lands on the valve, get it checked by a diving shop or the manufacturer. If the valve shears off, you will witness the launch of a blunt but very powerful rocket, the trajectory of which will be random and uncontrolled.

Airgun Cylinder:
Filling: Because filling cylinders is generally not performed by the diving shop but members of the public, the possibility of problems are much greater so carefully follow the instructions in the gun handbook.  
One of the greatest dangers is allowing the air from a full 300bar tank to equalize with a 200bar cylinder, this is surprisingly easy to do if attention isn't being given to the process. Most airguns on the market today have an upper limit of 200 bar, going over this, even to 300bar, may not cause total failure but could render the cylinder beyond further use.  
Once connected, open the tank valve slowly and by as small amount as possible, watch the needle on the tank gauge rise slowly towards the desired filling pressure. Don't instantly open the valve wide, this will generate a lot of heat in the cylinder and reduce its working life. A good indication of a correct fill is how little the cylinder temperature rises.