Try Art's Place: 931 Sunset Dr G, Costa Mesa, CA 92627. He repaired a small rust spot on my rain gutter and his pricing was fair. Quick turnaround too.
Another place that was recommended to me, but that I did not use, is Autobahn auto body in Costa Mesa. I think they are more of a restoration shop so they may be more expensive.
Unfortunately these rust spots have a tendency to keep popping up if you are parking outside with the marine layer and rain. The water will sit in the gutters while parked and find any nook to seep in. After a couple of years parked in the driveway, I ended up taking the garage spot for my cruiser.