I have a puddle of gas under the ATV each morning. I assume it's a sticking float. I was going to tear it down and rebuild the carb. What local types of stores carry rebuild kits? (How do I find them, assuming there are others beyond a Yamaha dealer) Are there particular brands I should look at or avoid? (sorry, I'm new to the ATV world).

It's a Grizzly 660, 2006.

Also, while I have this torn apart, what else should I be doing at the same time? I just bought it, so I have no idea when maintenance was last done. I'm thinking an oil change, air filter.. what else?

 

Usually this is a sticking/worn needle and seat if it's leaking from the carb over flow.Might be cheaper to get a carb kit on Ebay than from a dealer who would charge just as much for the needle and seat itself.
http://www.ebay.com/itm/2006-2008-Yamaha-660-GRIZZLY-CARB-CARBURETOR-REPAIR-KIT-YFM660-03-333-/331063400640?pt=Motors_ATV_Parts_Accessories&hash=item4d14ea98c0&vxp=mtr