Most everyone will have something in the closet that will work for a farmer character. The traditional look would be a plaid button-up shirt and overalls. You could get away with a pair of jeans, flannel, and a straw hat, as well. 

If you want to be more playful - you can dress up like one of the animals you care for daily!

Or, you can do a combo of both - wear the traditional plaid flannel shirt with overalls and a cow ear and tail set. 

You live once.  It's time to make your friends laugh! 

Have fun at your party!