If by plastic trapway, you mean the "unifit" adapter, then you are in luck. Toto makes a 10", 12" and a 14", so I assume the 10" will work, though you will have a fairly large wall/tank gap. You should be able to find one on the Toto web site or any of the websites that sell thier products. I would also assume wherever you purchased the toilet would have one as well.
Another thing you might consider, if the plastic trapway is a standard diameter, is to cut a 1' section out of it and solvent weld the two end pieces together with a pvc coupler. What the heck, you will have to buy a 10" adapter anyway, it might be worth a try. The adapters are about $50.00 as I recall, so a little ingenuity might be called for.
We'll see if that makes Terry cringe.