1st see if you can measure the dimensions of you powersupply
2 generally you can go to the home pages of various power supply manufacturers and get dimensions, maybe this can help, if not try calling a retailer and give them your dimenions and maybe they could help you also in trying to find one that fits... if not you may have to do a bit of modding if you have room
3 are you planning on overclocking this machine or not, what is the machine mainly going to be used for? gaming,office stuff,grannies e-mail? what is it going to have ? dvd,brand new vidcard, onboard vga??
4. a good rule of thumb is get the best power supply you can afford.
some good ps are as follows PCPOWERCOOLING(the best but expensive)antec,enermax,sparkle etc those are some brands you should be able to trust
hope this helps and good luck