Try a search for Zenith Cutter or inquire about Simmons Knives from Leonardi Manufacturing. I've dealt with both (not about Dosko) and they both are helpful and provide good service. The Zenith knives (at least some) are actually made in Vietnam. The Simmonds are made in the U.S.
Couplers and feedwheel motors are standard industrial parts-check out a hydraulics supply house. The rollers..... I don't know about getting a new one.... why not just work with what you have and weld, grind as needed?