Does anyone know where I can get the heavier thread for tying bucktail jigs?

Various types and sizes of tying thread are available pretty much everywhere; Cabelas, Bass Pro, Barlow's, almost every fly shop. What are you currently using and we can give you a more specific answer and recommendation. Bucktail isn't that hard of a hair, so, you don't really need the real heavy duty stuff that is needed for spinning deer hair.

Well, I just haven't tied anything with that thick of a thread. Size of thread for 3,4,5 ounce bucktails is what I'm talking about. Tying them for Stripers.

Bass Pro sells a thread called Big Fly.

Big Fly is a good option. I would avoid all 70 denier threads. I'd suggest starting with 210 or higher denier threads. The gel spun threads would work fine, but are a little bit of overkill for bucktail. Gel spun is often labeled "gsp".

Not sure where to get it in Texas but as others have said Bass Pro should have some though, maybe Academy though I do not remember if they had a fly tying section. If finding it is a problem there is a fly shop in Round Rock or you could go online to Feather Craft Fly Fishing, they have every thread under the sun and could advise as what would be best. I would shoot for 3/0 Uni thread or Danville's' flat waxed nylon 210 denier. You could even use single strand floss. The Uni Thread works the best for me. Bites into buck tail just enough without breaking and isn't as slick as the nylon or floss. Big Fly thread can be a bit bulky if your trying to keep everything neat and flat it also holds your whip finish much better. With Danville's' or floss a coating of head cement or Sally Hansen's is all but absolutely necessary. I use the Uni Thread on Clouser style flies and on buck tail jigs for my buddies here in Colorado. I would assume you are tying up bigger jigs on a size #6 or larger hook. If you were tying for crappie or other smaller panfish on a size #8 or below you could use a 6/0 or 70 denier thread.

If you want heavier/larger thread use rod guide wrapping thread in either D or E. E is the largest.

Polyester embroidery thread frm walmart works great. 2.59 for 300yd spool in your choice of colors

Yeah, go to any fabric place and get embroidery thread.