[Chrios]

I don't think you can by them by themselves...I had a cursory glance around when I lost the spring for one of my ARs. If you wanted a new one, you'd probably have to get an entire selector switch.

While I don't know for certain whether or not an M4 sized set will work, there's no harm in trying - either it does or it doesn't (assuming you have one handy). The good news is that metal ball bearings and little springs aren't controlled substances, so you just pick something up at a hardware store. Or disassemble a pen.

edit to fix horrid syntax