They are very hard to find. Always research availabilty of replacement parts, particularly consumables like lamps. Odyssea makes a killing selling these oddball fixtures, people buy them because they are cheap but then they find out they can't find the lamps.
T6 lamps are basically T5 lamps with T8 bases. If these are 54w lamps, then the fixture is equipped with a T5 ballast. You can probably crack the fixture open and replace the sockets with T5 sockets so they can fit standard T5 lamps into them. You may consider this option.
I'm betting you got this fixture from the "traders" or "Al's". You may be able to contact them and get an idea of where to go for replacement T6 lamps.
Just remember, they are odd lamps, which means you'll always be limited on lamp choices and availability. If you figure that you're going to get frustrated at some point and change out the fixture sooner or later, you might as well change it out now and avoid the expense and aggravation until you do.