Depending on the size/color/style, there are some great deals on
crocs.com. I just got 3 pairs for $25 with free shipping. I don't know how soon you need them, but you may have to watch a while to get the really good deals.
Otherwise, Amazon.com has a good selection and fair/good prices. A lot of styles are available from many different retailers so I can generally find something for the price I want to pay.