Is there any place here that sells them? Preferably aluminum or chromoly. I don't like buying complete bikes because I usually replace everything except the frame.

I found a cheap Flying Pigeon but at 20" the frame is a little too big and the steel is HEAVY. "Common" used loop frame bikes, such as the Raleigh tourist and its Chinese knockoffs, seem impossible to find in 18" or 19".

There are many US sources for new and completed loop frame bikes, but with the exception of Linus, I think they are all too far out of my budget ($300-400). I thought I could save some money by just buying a frame and outfitting it with loose components I already own.

Maybe they can help you search

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Vintage Dutch Style City Bikes - Custom Bicycles | Papillionaire USA

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Step-Thru Seven-Speed City Bike - Critical Cycles

they're meant to be pig-heavy.