Not any PSU would fit. I have a minitower from HP too and those are really tight. The size of the PSU should be 5.9"(W) x 3.4"(H) X 5.5"(L); 150mm(W) x 86mm(H) x 140mm(L) or smaller. Also, the PSU shouldn't have too many connectors because the case is really small. And make sure that the card isn't too long (10 inches at most, preferably 9 inches or shorter).
So this would be my PSU recommendation: http://www.newegg.com/Product/Product.aspx?Item=N82E16817256065
It's not exactly cheap but fairly good quality and is modular, which would be very helpful since the case is so small.