Hey man. So there's some good news and a lot of bad. First,
Your best bet for used parts is to be in town for FRANKEN-BIKE which happens once a month and is more like a swap meet.
Unfortunately, there isn't a co-op in Houston at all. At least not in the same vein of Gearin Up, Velocity or Phoenix. There is a "community focused" shop called 3RD WARD BIKE SHOP which sells used parts and bikes. But the prices arent all that great...but they arent set in stone either - so you might get lucky.
When a friemd of mine was planning a tour he had some luck calling a few shops and asking if thwy had a used parts bin he could dig thru. None of them advertised it but a few let him. Try Bayou City Cycles, Sun and Ski Sports and Bike Barn.
Best of luck...
Jonathan R Stafford
Native Houstonian, Proud Texan