Cool project! For the 30's-50's, consider eBay France. Shipping a bike here is ~$120, depending on the seller. You can find som really cool, old bikes there, and if they were on this side of the ocean, would be worth much more just because of stateside rarity. I've sometimes thought about taking an extended vacation and doing a European Picker tour, then ship them all back in a container. Do a pop up store in the early summer...
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If someone tells you that you have enough bicycles and you don't need any more, stop talking to them. You don't need that kind of negativity in your life.