There are very few heavy tanker positions available, and very few heavy tankers left. Your best bet at an entry position would be to seek out an Air Attack job in a Commander, either with Spur Aviation (Twin Falls, ID) or Ponderosa Aviation (Safford, AZ) to get your foot wet in fire aviation.
The largest number of air tankers presently are single engine air tankers, almost all Air Tractor AT-802A's. For those, you'll usually need to already have solid 802 experience, ample conventional gear (tailwheel) experience, and mountain experience. Not a lot of positions come open each year, but a few do. Western Pilot Service, Queen Bee, Evergreen, Henry's, New Frontier and a few other operators have exclusive use and call-when-needed contracts.
Neptune flies P2V's and the BAE-146. Tanker 10 flies the DC-10. Minden flies the P2, and shortly, the BAE 146. Coulson is just getting their C-130 online. Aeroflite operates CL-215's.
There are more heavy tankers in Canada. Good luck getting across the border and getting work in one, though.