First of all, replacement glass panels (OEM) are about 50 bucks each from the discount house called Rovers North.
Second, the channels that attach to the pieces of glass are press-fit on. Get some pondliner in a couple thicknesses from a home depot or lowes and experiment until you get a nice tight fit, then trim the excess.
Also, the whole unit comes apart by drilling out the rivets at each end of the inside track, and the 4 around the lock handle. Then some screws in the bottom of the channel and you are good to go.
While you are in there, replace the felts while you are at it, also about 5 bucks each from bargain basement warehouse Rovers North.
Lastly, any professional shop that cant figure out a LR sliding window must be staffed by morons.