West Marine, Boater's World someplace like that. They are Ronstan sail boat blocks, I have used them a long time on different wingers. There are different sizies and types, I believe the ones on my GU's are 400 series. You can also get them with ball bearings if you want to spend that much, I have some of both and wouldn't spend the extra for BB again, there's just not that much difference from the standard 400 series.
I would contact GU and have them send you replacement pulleys. I bought a GU when they first came out and had to go and buy aftermarket pulleys for 40.00 to ensure the winger worked everytime. I hear the new nylon pulleys work better.