Unless you're rich and have money to burn, you're better off using the Well / Firepower clear one. The upper and lower receiver are 1 piece and the shell splits in half, whereas higher end P90s (everything else NOT Well or Firepower), the upper and lower receivers are 2 separate parts. And I think you'd have a very difficult time modding that to fit a paintball marker.
I've saw a video on youtube of someone modding a Well P90 with paintball marker (can't find it now), and I don't think you'd be able to do it with anything other than the Well.
The only places I know that carried the Well P90 are S.I.R. (now Cabela's Canada) and they don't seem to carry it anymore, and buyairsoft.ca (
http://buyairsoft.ca/catalog/product...roducts_id=135), but it's been out of stock forever now. And as you can see, even those cheap guns are over $100.
So your best bet is to order from the US. Clear guns are legal to import, but it's very dependent on how the customs agent that processes your package interprets the law. There is a possibility it could get seized. But chances are, it will get to you.