I've always found that Drs Foster and Smith have really good prices for many things. Although there are probably some cheaper places, the price won't be that much difference. Plus Drs Foster and Smith have lot's of products with free shipping. Make sure you buy them from a reputable company. It also depends on the type you want and the weight of the dog.
I use 1-800 pet meds online. I'm not sure if the prices are the cheapest, but they are cheaper than any pet store I've seen. Sometimes they send coupons via email. They usually arrive within 2-3 days of ordering.