http://www.modellbau-seidel.de/
German Tamiya webshop, who has pretty much everything and also sells pretty much everything seperately, things like the WO Universal Shaft cross joint parts (which is really nice as those shafts have an off diameter) and rear axle washers. It's not the cheapest store but to be fair Rene Seidel does have everything Tamiya you might need and postage has always been pretty quick in my experience (to the Netherlands, though to the UK can't take much longer). Only downside are postage costs, it's a flat rate, regardless whether you order one washer or a whole kit.
The 19804655 axle washer is currently sold out, I'd just contact him and ask him about it
