Moses, the AFV Club tracks are sold as two separate sets. See:
http://www.clubhyper.com/reference/m3tracksta_1.htm
You may need to troll around the net for someone with stock.
The M4 tracks are Shermie tracks, a tad too big for your M3 Stuart - maybe an M3 Grant or Lee. Don't you just love the US military use the same vehicle designation for different vehicles, like the US had the M3 half-track too!
The Tamiya M3/M5/M8 Stuart series is old, lacks detail and are very innaccurate. The Tamiya hull is too short in length, so replacement vinyl from, say Academy's kit, will not fit. I think Fruil and Kasten make an indy set, but a lot of money to spend on the old Tamiya offering. The Academy M3A1 is not that bad but the Brit one has an innaccurate interior. The AFV Club one is supposed to be the best kit around and rather than shell out for indies for the Tamiya one, buying the AFV Club one may be more cost effective.