There's a growing market for World War II armored vehicle spare parts. Most of these spares, like tracks for tanks, are manufactured new. As the Baby Boomers (kids in the West born 1945-65) grew up and prospered, many sought to taste a little of dad's war stories. This led to a lot of the boomers using their wealth to buy, and often restore, World War II vehicles, aircraft, and even a few small warships.
The end of the Cold War in 1991, opened up huge supplies of World War II era Russian vehicles and equipment. Some of this stuff was still in use (sort of, it was stashed away as "reserves".) But there were still a lot of hulks lying about, and more stuff (including German gear) was found, well preserved, in lakes and marshes (due to new sensors for detecting these large metallic items.)
Eastern Europe was also a source of collectable German and Soviet uniforms and period military equipment in general. The demand was so great, that some enterprising folks began manufacturing World War II uniforms and military equipment.
Oh, and for those of you in need of new tracks for your M-4 Sherman tank, here's one good source; www.staman.nl