Think seriously about what you want from it.
As a sidearm pistols are a waste of money, extra weight and are almost never used.
As an alternate style they are great fun. Like pump play it takes a different mindset up against normal players.
Some cheaper paintball pistols are around, but tend to be cut down markers - such as the Delta 68 which is a Spyder design but using a horizontal spirng tube feed and a short barrel.
The two main paintball pistols are the Tiberius 8 range or the Tippmann TiPx, these are a matter of personal preference and how you want to play.
They are designed around 12gram co2, but can be adapted for air via a remote line.
On 12 gram I rate the Tiberius as best - the co2 goes into each magazine which means you use a lot of 12 grams, but can refill each magazine 3 times per 12 gram.
The Tippmann has the 12 gram in the pistol which means you do not need 1 12 gram per magazine, but when that 12 gram runs out you have to change it then, and could get caught out. With the Tiberius you manage when you change the 12 gram.
When using air the Tiberius magazines are adapted. This means you use one adapted magazine and would have to keep reloading the magazine with paint.
On the Tippmann the air goes into the back of the pistol so just carry as many magazines as you like.
Its also possible to make a mount to fit a 13ci cylinder to the pistol, e.g. under the barrel.
This works well, but looks pretty weird and is off balance.