In my experience it has always been Provident Metals. You get free shipping with any purchase over $100.00 and there stuff is inexpensive. You can also check comparesilverprices.com but watch out for shipping on the sites that advertise lower than Provident.
Provident and JMB are good defaults for low prices, but always be sure to shop around. It only take a couple minutes to check a few other sites. APMEX, MCM and others can have better pricing on certain items. And don't forget Ebay. The Ebay stores for many of these guys can have lower prices than what is available in their regular online store. There are also some large Ebay-only dealers, like Bay Precious Metals, that have some great prices.
I've bought from Provident, JMBullion, SDBullion, Bay Precious Metals, Apmex. All good, so I shop for best price. Also note discounts for check or wire payment. For example, JMB (and Apmex?) offer eChecks for certain existing customers -- lower prices with no delays for snail-mail check delivery.
While I prefer shopping online for silver because of low prices, I also like to purchase from dealers with good prices. Reason being if I am buying from them They will buy back at spot rather than under spot. This is one dealer I found. He sold me 20 rounds for $1.25 over spot each but his buy back is spot. The only place I have seen a buy back online is Provident but you have to sell them over $1,000.00 worth and who knows how much they might try to ding you if it's not it perfect condition.
