First off, dart molds are pretty easy to make out of plaster and generally last me about a dozen runs. Or, you could carve your own dart out of balsa wood and take it to a machine shop to have them make a mold for you. That might be pricey, I've never checked.

Lead sources can be found by looking in the yellow pages, try Industrial, or metals, or such. Usually you'll be able to ask around and someone can point you in the right direction. I had a source in Des Moines but I believe he retired. I still have about 50lbs in a bucket for future use.