It depends.
When APMEX has sales, they're a good place. When they don't, then often you can get better prices elsewhere, like Gainesville Coins, but not a whole lot better.
If you're looking for junk Silver, I think eBay is the best way to get it. Just be careful in your bidding, and be prepared to lose a lot of bids before you get lucky and snag some for a good price.