I go to different places for different leathers.
For vegetable tanned belt or strap leather, I like Crazy Crow. Their natural strap leather is more supple, easier to work with, and less likely to crack when doubled over than Tandy's belt leather. However, I bought some willow buckskin from Crazy Crow a while back and was disappointed. It was full of holes.
Tandy is the only source I know of for milled veg tan leather. If you have a project for which vegetable tanned leather is indicated, but you want softer, more supple leather, as for a shot pouch, milled vegetable tan is the way to go. I have not read much about other people using it, though.
It is tempting to buy "remnant" bundles, but I would be cautious. Some places will include some usable material. Tandy's remnant bundles have been mostly scrap, with hardly anything of usable size.
I like Centralia Fur & Hide best for rawhide.
I have gotten "commercial brain tan" leather from The Leather Guy. He sells it by the piece. The leather you see pictured on his website is what you get. I have also gotten some chrome tanned buckskin from him that was very satisfactory.
Wandering Bull has brown pre-dyed strap leather that I like a lot. They will sell it as straps cut to width, or by the square foot. The price is reasonable.
That's my experience, but you'll encounter lots of leatherworkers on this forum, and they all have valid recommendations.
Notchy Bob