I've used Tulving.com, APMEX.com, and Bulliondirect.com in the past.
I like APMEX and Tulving the most. Tulving is better for bullk purchases, APMEX for smaller or 1-2 coin purchases.
BullionDirect has a trading site that you can buy/sell/trade with other members. It's nice, but their delivery prices and sales prices tend to be higher.
You can try Ebay too - but caveat emptor. I haven't been burned yet, but gold and silver is getting hot now, and may be prone to mischief by the less reputable.
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Caveat Lector! The opinions expressed in this post are those of FBH and no other, not even my employer!