You certainly can't go wrong with OEM accessories purchased from the manufacturer, but this is also the most expensive route to take. As far as accessories, places like CDNN Sports, Sportsmans Guide and Midway USA are very popular. Of course, the major sporting goods companies like Cabelas, Bass Pro Shop, Dicks Sporting Goods (to name a few) can be good places to check out, but they will, in most cases, NOT be the cheapest, but probably a little better than buying from the manufacturer.
I am sure you will get many many more responses.
You will probably also want to check out some local gun dealers, especially if, I mean WHEN you plan on buying more guns. Also, if you buy guns online, they will have to be shipped to a dealer with an FFL (Federal Firearms License) so it pays to become friendly with the smaller shops in your area.