There's nothing special or magical about Mesa branded tubes. They've just been checked and sorted and relabeled. BUT one of the checks they do is for current draw at operating voltages, and this is critical. Since Mesa amps do not have a bias adjustment, you must be sure to use tubes that have current draw in the correct range or you can wreck the amp.
I have used, and am currently using, non-Mesa tubes in my Mark IV. My amp tech has a tube tester which I used to test some Mesa tubes - the 6L6 STR430s, as well as the EL34 STR 447s, so I know what the current draw should be, and I selected non-Mesa tubes that have the same or lower current draw.
I've heard that if you order from Doug's tubes and specify that they are for a Mesa, he will select ones with low enough current draw.
If you decide to use non-Mesa tubes, its a good idea to proceed with caution. After you install them, watch carefully as they heat up after you turn the amp on, then start out with the output and power low and work up to your normal performance volume in baby steps. If they start redplating, or getting discolored, you know you have trouble. Also, if there are noticeable changes in tone, something might be amiss. Good luck.