I agree with Marc411 that a place that does monogramming/engraving locally can probably do it. I've never got a tank engraved, but my husband has taken lots of random metal things to our local trophy-making place (he and his friends have a running joke about giving each other engraved stuff, and I've seen them have everything from a trainer hand-grenade to a flashlight engraved). Might be worth at least taking it in and letting them look, if there is a place around you that does it. Most of those type of places can engrave things like flasks, doesn't seem that much harder to engrave a stainless steel tank.
Also, if you're crafty and artistically inclined, you might be able to do it yourself. They sell engraving tools that are battery operated at most hardware stores, for etching your name on your tools. They run about 20 bucks. If you try it and are good at it, you might could strike up a nice side-business when other vapers see your cool customized tank and start begging you to tell them where you got it.
