My local place is American Paintball Tank Hydrotest. Paintball Hydro Test service
But, you run into the same problem with shipping. Less than $20 per hydro is great, and being local to them, I can take a couple of tanks in, hang out in town for the day, and drive home with fresh tanks. But when you factor in shipping, I highly recommend hitting up something local to you. But do what you can to remove your own regs, or have a paintball shop do it for you. Most places don't have a clue about removing paintball regs, and they tend to damage them. Not alot, but there are more tanks that get sent in for hydro that they get a damaged reg in the removal process than the tanks themselves actually don't pass hydro.
As to a CO2 tank, it isn't really worth it to get them hydroed. For the same price, or maybe a little bit more, you can buy a brand new 20 ounce CO2 tank. But if you can find a Blue Rhino that still does the CO2 tank swap program, go for it. The people that work at the place that does the swap don't have a clue about hydro, so you would be swapping an out of hydro tank for one in hydro.