A DIY sump is pretty easy to make. All you need is a tank, acrylic, and silicone. I'd recommend getting your acrylic (not plexiglass) at a plastics shop if you can. They will cut it for you as well. You can buy silicone cheap by going with GE Silicone #1 (Do not buy #2 as it has mildew resistant chemicals and will kill your fish).
This vid gives a decent how to on sump making. Good luck!
Reef Tank D.I.Y. Sump part 1 - YouTube