I get kits from many sources....local hobby shops, online vendors, ebay, local shows..
As far as the online vendors go, my favorites are Evers Toy Store for Revell, AMT and Lindberg kits (great website, great customer service) and Hobby Link Japan for Fujimi, Aoshima, Tamiya, etc. I've used Model Express occasionally also. I used Hobby Heaven as well before they folded. For supplies I get most everything from the local shops.