I think almost any PSU will fit, the problem would be for the cables to reach your MOBO and stuff.
For example, I have a Gigabyte S3 Mobo and a thermaltake PSU, and since the PSU goes way down and the 4 pin connector (I think its power for the CPU) is located way up, the cable barely made it. I mean BARELY. In fact, I had to force it a little bit.
By the way, I got a PSU with fans in front and back 'cause I think i would work better in P180.
For example, will a Silverstone OP850 fit? How about an 850W Enermax Galaxy?