Hi,
Just a thought - if you are having real trouble finding a small sprocket for your TMX, remember that the same sprocket was used on the 90 Trail, 90 Mountain and 100 Trail. Only the large sprocket varied between these models. Also, the rear sprocket used on the 90 Deluxe, 90 Sport and 100 Sport models is the same size as the TMX item - they just lack the two sets of holes to attach the large sprocket.
So, if you have your old TMX sprocket and a friend with workshop facilities, it should be possible to use the old sprocket as a template and drill a 90 Deluxe sprocket to fit.
Graham