Well, as your Tx can't be connected to your PC you could do what I used to. Get a Y-splitter joystick cable for the PC that allows you to connect two joysticks to the single game connector on your sound card. Then get two old (preferably identical) joysticks and, if possible remove the return springs on the vertical axis of one of them. Mount the two side by side (the one without return springs on the left to be like a "real" throttle) and voila! you have a very cheap and cheerful R/C controller that'll work fine with FMS (cos it has a joystick control option).

However for a bit extra why not get a copy of "Piccofly"+controller which basically is a ready made box a bit like I just described and you also get a pretty good heli simulator (better than FMS) into the bargain.

Cliff