You can get them mounted at any tire shop. Balancing presents some interesting challenges. Traditional wheel weights don't work so hot, those tires are typically so out of round, that they are ineffective.
The popular idea now is to use balancing powder. It falls into the voids, and through centrifugal force, will move into the out of balance ares of the tire making it balanced. It's spendy at ~$10 per tire. You can get it at a tractor trailer supply.
Some folks use sand, playsand, to do the same thing. Any moisture in the compressed air though leaves it kind of clumpy, and it can act funny.
Another popular idea is to use BB's. Regular old BB's that you get at any sporting goods store, I'm told that one carton (~1500) BB's per tire, does a good job. You'll hear them when you slow down, and it can be fun to mess with any new passengers while they plink on the wheel. (you're not going to hear them over the sound of the tires when you're moving)
I've heard of using golf balls, but I don't see how they can be that effective, they're too large I think.