There's seller on Ebay that sells used cabs sometimes. Right now he has one for a 743 and one for a 843 for sale. You can check out his stuff at http://search.ebay.com/_W0QQfrtsZ50QQsassZthesafetydepot
I've puchased from him before and his shipping is reasonable. His selection of Bobcat parts varies from week to week so you just have to keep checking.