Why did farmer john ask the supermarket manager where the overalls were?

This is a pun, or a play on words, meaning that it should have a humorous effect because of the way that two words that sound the same actually have different meanings.

So, why did farmer John ask the supermarket manager where the overalls were? - Because the store claimed to have the best overall prices.

Overall prices doesn't refer to overalls, but rather to prices in general.