if you're a DIYer, then Tony's suggestion is the best because you get to build it the way you want it - dimensions, zones/comparments, etc.
Buy someone's used sump... scour the boards, TRT, RC, MR, etc...
You can get a used glass tank and silicone baffles to it...
pay someone to build you one - most expensive route since I'm assuming you'll want at least 100g sump... look to pay at least $600 going this route...
use a
rubbermaid stock tank (100g or larger) - not the best looking or functional w.r.t. baffling but definitely can get you more water volume... a new one can set you back at least $150 ...