Your local coin shop is a great place to start, especially when buying small amounts. Coin shows are also a good place to scoop up some deals.
There are plenty of online dealers that are reputable such as APMEX, Gainsville, Provident, Silvertowne, and Tulving to name a few.
Check into the laws in PA. Some states charge sales tax on PMs, some don't, some do if it is under a certain amount cash price but don't if it is above.