I have been designing a PCB for the past month or so, and I finally got the courage to order it. I decided to go with OSH Park as it is low-price low-quantity, which is what I am looking for. While that's in the works, I got the idea of ordering a flex version of the PCB I made. Does anyone know of some low-cost low-quantity fabricators that can do flex (or rigid-flex)PCBs? I have been looking around and so far I haven't found any.
Seeed Studio will do flex boards. https://www.seeedstudio.com/fusion_pcb.html (https://www.seeedstudio.com/fusion_pcb.html)
Be prepared for sticker shock, though - 10 boards 20x50mm seems to run about $175 (~30x more expensive than FR4.)
Good chance that flexible PCBs are so little in demand that it requires special setup to make your prototype. I've seen them in quite cheap products so mass production (with that setup cost spread over thousands of units) is much cheaper. They're indeed VERY expensive in small volume.
Part of the reason they can make the regular <100x100mm PCB so cheap is because those are panellised into much bigger boards together with orders from other people and then mass produced, no special machine setup to be done.