Below is a list of suppliers I have personally used. There are many others, so don't feel to limit yourself.
Tandy Leather
Rocky Mountain Leather Supply
The Buckle Guy
Made on Jupitar Leather Lap
Joann Fabrics
Big Duck Canvas
Leather Machine Co.
The Thread Exchange
Superior Threads
Purse Supplies R Us (Etsy)
Peggy Sue Also (Etsy)
Zipit (Etsy)