I guess that's true. Among the more notable recent small shops, Velo-Orange and Rawlands do not build frames. They design frames then find a fabricator off shore, with prices (for steel) in the $700 to 800 range. Boxdog Cycles has designed the Pelican but has it made by an excellent American builder, Ahren Rogers in Madison, WI, for a price around $1300 if I recall. Similarly Boulder Bicycles, builds the Boulder Randonneur onshore for about $1600 (all my numbers could be out of date).

What's your intended budget, and do you have any specific geometry goals or restrictions? There are a lot of good used frame/fork sets out there.

If your main point with flex is operating integrity (freedom from rubbing, et cetera), do you spin or mash? Also, perhaps a few more millimeters of BB length would provide adequate chainring and crank arm clearance.

Serotta, in the '80s, produced the Colorado line, which had a larger tube diameter at the BB to stiffen it. They turn up on Ebay fairly regularly, for sometimes a lot less than $1000.

There are also genuine construction differences in steel frames, in addition to tubing diameter differences. One principle is to design the BB shell and to miter all the tubes that enter it (especially including the chainstay) so that they are brazed not only to the BB shell but to each other. One example of a frame where this was not done is the Peugeot UO-8, known as a flexible frame. Many frames of days gone by; in my experience the California Masis, Mondonico, classic early '80s Treks, English Woodrup, and Raleigh-Carlton Super Course) were built this way and at least the Masis, Woodrups, and Mondonicos were known quantities in the peletons of the day and hence able to withstand professional strength and loading. My own bikes do not show such behaviors, but that could be just my pedaling or lower strength. BTW, all these frames I've just mentioned have the traditional tube diameters.