According to some studies (that I cannot find links to right now), a lot of pet vitamins and many foreign and 'off-label' brands of vitamins do not have what they claim to have, and, within reason, 'vitamins are vitamins'. That is, as long as you are using a good brand, the major differences are the exact ratios of nutrients- which probably don't usually need to be real exact.
So, what I do is to use some crushed human multi-vitamins at the rate of a small pinch a couple times a week for 4 torts that are a year old. This way, I know they are reasonably fresh since I take them too (a couple tablets a week). Heck, as little as I use for them, I can crush a bit of one and still have plenty for me for the day!
The method Mark(madkins)is using is about the only one I heard of people using to any extent. There are reptile multivitamins,but I have never used them.