I have no idea how you are going to be able to squeeze a 180 section tyre on to a 75mm wide rim. But that's not my problem.
My Kubota Tractor has 8 [inch] x 16 front wheels with tubes. The tyre shops just use a light truck inner tube and these are available at most car tyre shops. I wouldn't worry about the profile [height] of the tube as tubes flex to fit inside the tyre and the rim. If a light truck tube is too big to fit then go to a car tube.
Provided the tube has to expand slightly to fill the void between the tyre and the rim it should work fine. Too large a tube will cause problems if the tube has to be folded to fit inside the tyre - you tend to end up with punctures where the folds occur.
Bookmarks