I've bought all of my silver rounds through APMEX on line. They're a company out of Oklahoma.
If you buy at least 100 dollars worth at a time, they ship it for free.
The main reason that I use them is because they accept paypal and credit cards. Of course, there's a bit of a premium tacked onto the price because of the credit card fees that they have to pay for taking credit cards. They seem to be a reputable company.