The TSR game is pretty rare, but not outrageous. Can't remember who, but a Google should find them for you. There is a distributor out there that still has some new ones. The 1987 re-print is not really hard to find. The project was delayed a number of times, and I do not know what its current status is. Guardians of Order were supposed to be putting out a new version of the game.
Of course, there are also a bunch of fan produced conversions of the setting to about any of the various generic systems out there that you can think of. Barker also wrote a few novels for the setting. I'm not particularly familiar with these, and I don't know that they were ever widely distributed. There have also been a couple other official rules sets for the game. No changes, just a straight-up reprinting. There was a re-print of the TSR version of the game in about 1987. I believe TSR stopped supporting it around '77 or '78. Judges Guild put out at least one adventure for it. I believe there were miniatures rules for it, as well as a board game based off of the setting. There wasn't a whole lot of support for the game other than what you saw in Dragon. And, as it's a D&D variant, it's arguably the first D&D world ever published, too. Tekumel is therefore the first rpg role playing world published. That was something that was just not done in 1975. It's most remarkable feature was its fully developed background and campaign world. It was very much based on D&D, with a few twists of its own.
TSR put EotPT out in 1975 at about the same time as the Greyhawk and Blackmoor supplements to D&D. Lothar TVNI wrote:About when did this game lose support? Was it ever very popular?