I doubt whether many parts would be available for this machine. The problem is likely that no firm dealership arrangement, or parts backup, has ever been established in the U.S. for these tractors .. and the few that are in the U.S. are likely to have been private imports .. or the factory offloaded a few DT-75's to a Belarus tractor dealer, to see if they would sell.
You could try a Belarus tractor dealer in the U.S. for parts availability for a start .. but it's likely you'll get blank looks, or a straight "nothing available" reply. Your next source would then be either a British Belarus dealer, as these crawlers appear to be sold and supported by them in the U.K.
As a final step, you could try contacting a Ukraine company such as Agrotech Export, as listed at the bottom of the page on this following website ..
www.exportbureau.com/search.html?search=tractors
Agrotech supply what appear to be aftermarket parts to the likes of the DT-75 .. but not every part will be available. Agrotech would be able to supply basic engine parts as well as some other major components. They have a good website and are set up to deal with Westerners.
The problems you are facing are many .. and they range from a tractor that is regarded as an obsolete model .. to language difficulties (the VTZ factory website does not offer an English version, or offer any representatives who speak or write English - whereas Agrotech do) .. a lack of dealerships .. and a simple lack of parts supply, even from the factory.
The Russian tractor factories also build military equipment such as tanks and tracked vehicles, with many interchangeable components between the military and civilian equipment .. and thus have a tendency to be secretive and suspicious as well.
This policy is a well-honed Russian initiative designed so that tractor production can be dropped quickly in favour of military production in the event of sudden war.
I wish you luck in your search for parts, but I wouldn't hold out a lot of hope for good parts availability. As a final recommendation, maybe trying to contact some of the owners of British DT-75's to see what they do for parts, would be a wise step. There are quite a number of these tractors in British collectors hands.