When I was trying to get mine fixed most shops didn't want to touch it because it was aluminum and they either didn't have the tools, the equipment or the knowledge to weld aluminum. That is why I sent it to Myle'rs to get it fixed.
I used these guys; Myler's Radiators , for a radiator repair on a Honda Hurricane and was very happy with the results, and the speed at which it was done. It may be an option, all you can do is call and find out. It's tough to find someone who is competent to weld aluminum radiators.
Well, now I'm not sure sure about the "qualifications?" They did pressure test it the first time they repaired. As my cap was the original from 6 years ago, I changed it and then the leak started to show. Maybe not used to having the pressure again? not sure
NEVER NEVER NEVER USE A STOP LEAK ANYTHING UNLESS YOU ARE STUCK ON THE ROAD, AND THEN FLUSH THE ENGINE AS SOON AS POSSIBLE AND GET THE RAD FIXED AT THE SAME TIME.
If you can find the leaking spot, your local welding shop should stock aluminum brazing rods. You could should be able to do this in your garage with a MAP gas torch. The only problem might be that the leak is on the side of a tube and not accessible.