Well as having my own TM M14 and it being in semi only. I now have the Issue of repairing/replacing the Selector arm that was broken when I bought it. I have found a couple replacement pieces one from Echo 1 as seen below and the other from Pro Arms. Now both are different materials but one offers full replacement while the other is just a steel counterpart for one side.
Echo 1. Location (airsoftgi.com, wisconsinairsoftwars.com) Approx $10
Proarms Steel Selector. Location ( Uncompany and airsoftshop.com) Approx $38
Its the best parts I could find. Also the only parts I could find. The Echo 1 is a great full replacement but if you want that extra support then the steel one is even better if you did manage to take the old one out with out breaking the other side.
Any more info would be great if someone found a different place to pick them up and a better cost.