I'm sure you could find an aftermarket turbo for the 944.
But there are some important differences between the standard 944 motor and the turbo (951) you should look into. I don't remember the exact details, but I recall the 951 had a different head with stronger valves and maybe revised porting. The headers were different too, as was the tranny (beefed up all the way through). The engine management was also different (obviously), not to mention the added oil cooler and intercooler.
So don't think simply putting the turbo on will automatically give you power levels on par with a factory 951. You may need to do a lot more work to get it there. Monster7 is probably right... you'd be better off finding a real 951.
M