Blowback armory as suggested deals with blow back weapons. They have a decent amount of parts and accessories that most retailers don't often carry.
I recently picked up a bunch of mags and parts from blowback armory. I received an email from them stating that the number of mags I ordered was not in stock (no surprise, I ordered a lot) but they would be shipped to me in two days. A few days later the box was on my porch. Everything arrived in good shape and as I ordered it.
I would not hesitate to do business with them again.