The manuals for the version of that stat I've checked (may not be your precise model) do mention pairing. Generally its almost impossible to buy the transmitter part only, so you buy a new one complete.
Have you tried moving it much closer and seeing if it communicates, or is the thing just plain dead?
One possible answer is, if you find the pairing instructions online (possibly by just googling "pairing xxx model stat" to buy a complete new one (not that cheap, I know) and pair the new one with the old receiver. Replacing the receiver is very straightforward indeed, after turning off the supply to the controller, obviously... (unless, of course, like the twat who installed most of my CH not long before I bought the house, he managed to fit the stat receiver hard up against the wall, and how do you remove the faceplate? By inserting a screwdriver into the side of it, yes, the side that is hard against the wall...) Mine is on its last legs too, such a huge hysteresis of about 8 degrees, I'll replace it completely with a programmable one.