just buya new heater core. they arent expensive. dont waste your time looking for one in a junkyard, chances are that that one is bad as well. i got mine on Ebay for i think $20. it wasnt easy to install, but the job got done. took the better part of an afternoon to do but yup, it saved alot of cash.
I had the same problem and I just disassembled the heater box. The core was leaking in several places. I ordered a new core as well as a gasket kit for the heater box. It`s the perfect time to change all the gaskets and clean up the box. A little time, cleaner and paint and it'll look like new!