I wish I had a great leather supplier. If you know of one, please tell me.
I have had good luck buying leather on eBay for a buck or two a square foot. Several times, I bought leather on auction from Brettun's Village Leather. They also sell directly, for reasonable prices.
For tools and supplies, I buy from The Leather Factory, which absorbed the old Tandy Leather franchise some years ago. They supposedly have a discount program for SCA members, but I haven't tried it.