I bought a SLR-106F. Now I'm wishing I had a rail on in. Can anyone recommend a good gunsmith to put a rail on? How much did they charge you, if you don't mind?

It's fairly easy, you can do it yourself. Got one from Tantal years ago for my slr105. There is a rail "kit" that comes with 3 button head hex head screws, instructions and wrench. You remove the trigger group, line up the rail parallel with the top of the receiver, drill the 3 holes and screw it in. Google mpk-02, collectorssourcedfw.com.  You could probably also use the riveted model rail, tap the holes and get 3 screws too if really in a jam. Hope this helps.

I know it is not a side rail but maybe give an Ultimak gas tube rail a try. I actually like mounting a sight to that better than the side rail.