Ya best case is a little vague to say the least. Be a little more specific in what kind you want - minimal, maximum protection, hard silicone, soft silicone, slider, snap on... you get the point. I've got literally tons of 3gs cases im sure I can find something for you.
Also if your buying online, unless something is rare or exclusive I would have to recommend ebay. (assuming you have a credit card/paypal its pretty easy to set up and definitely worth it)