They do exist, but not for all kits. AFV Club makes good rubber bands for a variety of Shermans and Tigers, and so do DML. Can you let us know what kit you're building? It would make it easier for us to point you in the right direction.
If nobody makes after market tracks for what you're building, you may well be able to trade for them. I have to believe someone in this community has a set of whatever you're looking for that they haven't used.