Firstly think twice about what you are doing - for 'practice at home' I read 'playing paintball offsite'
Consider the land available to you as to whether it is safe, legal (no shooting within a specified distance from roads) and won't get complaints from the neighbours
You could opt for some extra paintball cylinders (only about £30 each) and fill when you're at sites
For a scuba fill system you need:
Scuba cylinder - note that 3000psi is 206 bar, 4500psi is 310bar
A cylinder capable of 4500psi fills will cost more £s
Be aware of 'land only' scuba systems, these have a different regulator to prevent use in diving
The advantage of this for a fill system is that scuba cylinders have a slightly stricter testing regime. Even if you are using a dive cylinder for paintball fills no-one will fill it without complying with the test regime
You then need a paintball fill rig to go between the scuba cylinder and the paintball cylinder fill nipple
Everything you need is here:
http://www.lips-paintball.com/acatalog/300bar-12ltr-Dive-Tank-with-INTEGRAL-FILL-STATION--221.html#.UlfBPpm9LCQ
Going back to pressure options of 3000psi and 4500psi
You will only get one fill to full pressure, the scuba pressure goes down, the next fill is slightly less and so on
Restrict your fills at the start and you get consistent and many more fills.
Take a 232 bar cylinder. Don't fill to 3000psi, stop somewhere between 2000 and 2500psi
Take a 300 bar cylinder and fill to 3000psi
http://www.scubatoys.com/paintball/scubafills2.asp
Before you use a scuba fill rig get shown how to use it properly and safely
As a minimum get a general air safety brief first, eg an air pass session and read the ukpsf hpa 1 handout
http://www.oaklandsfestival.host56.com/web_documents/air information.pdf
http://p8ntballer-forums.com/threads/a-bit-of-info-the-the-experts.145017/