The Irvine Farmer's Market is on Saturday and is about 30 minute drive. It's in the parking lot of the business center across from the UC Irvine campus. It has great produce and vendors. Cal Poly Pomona has a booth with the produce grown on the university farms...fresh squeezed orange juice is the best from them.
There is a market in Buena Park on Saturday that is smaller but closer than is mainly fruits and vegetables. There's a newer one at the Great Park in Irvine on Sundays. If you want to include a little sightseeing with a trip to market. The Great Park has a balloon ride similar to the one in WDW Downtown Disney AND it's FREE. On a clear day, you can see all of Orange County and to the ocean...not WDW, but again it's free.