If you have a "pre-built" computer (HP, Dell, etc.) you'll have to do some research first. If you are only changing the PSU you may be forced to buy your replacement from the systems mfg (and that will be expensive). Some of the builders have proprietary PSU's which would require you to use their specific units only. If you don't have a pre-built it will still be easier for you buy a new case than to find a PSU that will fit yours. At least your selection of cases will be greater than the PSU's that will be available to fit your case. If you tell us what you have we might be able to help.