If the core is leaking, any reputable radiator repair shop should be able to leak test it and solder it up if needed.
Avoid Bar's Leaks or anything similar at all costs. There's no such thing as a "mechanic in a bottle", as it doesn't properly fix the root cause while it DOES risk creating a bigger problem. The ONLY time I'd use it would be as a temporary fix to get you home if you were stranded out in the middle of nowhere and had no alternative.