Try to search for local Theaters and contact the people who work on productions there. The season is pretty active around this time and if you live near a major city like New York, there should be an abundance of theaters that have old costumes laying around or at least can point you in the right direction on where to find them, especially for period pieces that require older fashions. Search for local theater groups in your town as well. They may already own old costumes that you may be able to borrow. You can also try stores that sell halloween costumes or costumes for masquerade balls.