Many of your small town weekly papers have ads for people wanting to buy used broken mowers. They sometimes are a good scource for carbs as most will save them while scrapping bigger things like rusted out decks etc. I have had good luck finding Kohler parts this way.
I have an 11 horse briggs (400CC) on a riding mower. Not sure whats wrong with your carb but mine has at least 15 years on it. Rebuilt the mower deck but the motor only required periodic removal of the float bowl to clean out gunk.
im not sure if your carb is worth rebuilding or not but if so i have sent several carbs to a guy i know who does just that. his website is carbrescue.com I ship it out to him and he takes about 5-7 days. he has completely rebuilt two small carbs for me, both for under $100