I'm all for reinventing the wheel. Each time it has been, it's become that bit better. If you think doing so is wasting programmers' valuable time, then don't ask the previous inventor to value the time he put in at zero.
The best chance of getting a source released is to promise the inventor that you'll make his wheel that much better
Another is to convince him he might die tomorrow and his sources should be preserved for posterity. Just try to make it not sound like a threat