Do you have the shop manual for your motor? If not you can go to www.YamahaPubs.com
and order it. The factory shop manual gives you instructions on changing the impeller, as well as proper torque specs and guidance on most other repairs. That would be the first place I'd start. You can order all the parts or the whole waterpump kit through iboat.net, they have pretty good prices on everything for your Yamaha.