There are other sized power supplies. Most of those are either SFX or TFX sized. You do NOT want either of those... They are much more like the shape of a loaf of bread than the square power supply that you want.
Make sure what you buy is ATX sized.
And if you want advice on what you are thinking about buying, just ask us. There are very high quality power supplies out there, and there is absolute junk power supplies, and you would often be surprised by the name brands on some of the junk ones. So please come ask us before you buy a power supply.