CMK makes vinyl tracks for the Pz IV. Eduard makes photo etch track for the Pz III, Cromwell and they used to make a set for the M3A1 half track, but that set is OOP. Extratech makes PE Pz IV tracks as well.
Part from Poland has the most comprehensive line of PE tracks too. I have most of the above mentioned sets but have not tried the Part tracks. I do have other Part PE sets and find them of superior quality.
You can get them at http://www.squadron.com in the US or http://www.tracks-n-troops.com/ in Europe.