I'm just starting to research postbacs myself, but as far as I know here are your options near Houston, Texas for completing science prereqs, specifically, since that's the boat I'm in.
1. UH - no formal postbac program, take classes as a postbac
2. Lamar - formal premed postbac, a premed advisor from another school did not recommend
3. Univ North Texas - formal premed postbac, same premed advisor did not recommend
4. UT Austin Extension: no formal program, take classes at night, inexpensive, solid option
5. Rice Class III Continuing Studies: no formal program, probably most rigorous science classes, very expensive
6. Houston Baptist University: no idea
7. Texas A&M: no idea
I've been researching for 2 months - that's what I've found. Please let me know what you find.