Looks like a outfit call California Veterinary Supply has some good prices:
http://www.calvetsupply.com/category/s?keyword=antibiotic
I’ve purchased from them and was happy with everything. At least as much as you can be without actually taking any of the product they delivered.
From what I’ve read you actually need a few different ones. Different drugs for different problems. Dosage is a big question. I’ve also purchased a physicians desk reference and a nurses drug manual to help with that.