Let me check my Rolodex...
Weaver leather, Tandy, brettuns village, seigles of CA,
Zackwhite.com are some of the big ones.
Craigslist can be useful for upholstery leather.
If your in the buffalo NY area go to Stefan's leather on mail st.
The shop has a very steam punk feel and has been in business sence 1851.
Spotted pony traders and real leather people are great to deal with.
Moscow hide and fur is another useful seller specializing in hides, furs, bone, horn, and antlers.
you can try eBay but its a crap shoot.