http://www.tankworkshop.com/newshop/
Here's their link; they list 110 products, but older stuff made for the older kits isn't listed.
No one wanted to spend the big bucks on aftermarket items for an ancient kit when a new, modern kit would cost less than the aftermarket items needed to bring that old kit up to modern standards.
In other words, why spend $40 on Fruil tracks, $20 on PE, $15 for an aluminum barrel and $40 on resin update sets for a kit worth $15 when the latest "Uber kit" runs $50, includes individual link tracks, some PE, metal barrel and is a dream to build?