Goto Robbins Napa or Suburban and buy the rebuild kit. They are easy to do.
Someone told me that Suburban was awfully expensive for things, but I never noticed that myself. The owner is Litchfield Fire Chief Tom Schoefield -- nice guy. I'd always recommend talking to him, as he's pretty knowledgeable.
Napa got me out of a jam on the first storm, making a new hose for me. It seemed as they had a lot of various plow parts. The hose was very reasonably priced, I might add.
Don't know too much about Donovan's. I went there once during a storm a few years back, and the replacement pump with labor was pretty expensive. They were a bit more expensive for a new Fisher installed, and they weren't accepting credit card for that (2 reasons I didn't use them). I definitely recommend EW Sleeper in Concord (obviously not convenient to many of you) for sales, service, and parts.