Usually the air tankers are contracted civilian pilots, handled by the US Forestry Service. It's not usually a military job for the water bombing. That doesn't mean they can't, my old unit trains heavily for helitack work with dipper buckets from UH-1's back when I was in, and UH-60's now. But it's a very different kind of flying that you need to be properly trained for. Your average cargo pilots usually don't train for the NOE flight in mountainous terrain that it requires. But the air tanker guys doing it in something like a DC-10 or a Martin Mars? That's some damned impressive flying!
The air tanker services are suffering a bit for airframes these days for a couple reasons. One is that a couple years ago they had a few aircraft break up in-flight, so the inspection process was made more severe and a lot of aircraft were pulled out of service. Then the economy problems hit and the government money dried up a bit, which cut back on them a bit further. So the dedicated air tanker services aren't as well equipped as they were a few years ago.
The C-130 system is actually pretty neat, assuming it hasn't been changed. They don't use dedicated aircraft, but it's a palletized system that was originally developed by an Air Guard crew back in the 60's/70's. Slide it in, fill it up, and ANY C-130 can be an air tanker in just a couple hours. But since it's not a primary mission for the military, they probably only have 8 kits.