Where is the best place to purchase small parts like the pins, springs, and bolt release parts? I just got back on a project that has been laying around for a while and seemed to have misplaced a few items. I am going to check out Brownell's first.
Midway or Brownells like bohem said. Prices vary between each company, so you'll have to shop around between them for the best deal. Also, don't forget the MidwayUSA discount codes that can be found here (usually around 10% or $10 off $100, $20 off $200, etc.). Those discounts may swing the best deals toward MidwayUSA all things being equal in terms of having the parts you need in stock and ready to roll.
midways website is also easier to use and find the parts you need. I prefer midway. I just irdered several smallparts for a remington 1100. Got my order in about 3 days.