There is considerable demand for vintage flexible nibs these days and I think that mostly applies to Swan, Onoto and Waterman. Indeed, demand for flex seems much greater now than it was in the heyday of the fountain pen. I do get flexible and semi-flex nibs, probably at the ratio of one to every three firm nibs. Not every flexible nib is fine, of course, the size most required, and not every one has the snapback needed to make flex really usable.
A common request is for a fine or extra-fine “wet noodle”. “Wet noodle” is a problem as a description. Everyone knows what they mean by “wet noodle” but no one knows what anyone else means! I take it to mean a nib with line variation at the outer edge of what is possible, easily achieved with very little pressure and with instant, elastic snapback. It needs to have consistent flow that will keep up with the line variation. I can pass whole years together without seeing such a pen. Those pens do appear in Swans but are more common, I think, in Watermans. The only way to tell if a pen has those characteristics is to write with it. Nib shape is no guide.
Historically, writing that made use of strong line variation predated the fountain pen and indeed, had gone out of fashion in the latter days of the dip pen, replaced by an easier, more rapid business style that paid only slight attention to light upstrokes and heavy downstrokes. It has been revived by calligraphers and those ambitious to be calligraphers in recent times. The demand for modern flexible nibs has never really been met though it seems closer now than ever before. It is constantly said in the fountain pen discussion boards that for true flex you must go vintage. That’s correct, of course, but it is sad that vintage pens are sometimes only appreciated for their nibs and vintage nibs are sometimes transferred into modern pens. In truth, no fountain pen will ever equal the line variation of a dip pen or even a quill, both much cheaper methods of achieving impressive flexibility.
I’m not trying to discourage those in the quest for their ideal flex fountain pen but I do wish to impart a realistic view of the difficulty. Some eBay sellers of unrestored vintage pens make much of how flexible the nibs are. Those pens tend to fetch a high price, often a price beyond what my customers would be happy to pay. I have to take pot luck among the rest and sometimes I get lucky. That is part of the service I provide. I don’t charge extra for flex. I apply the same margin to all my pens. Thankfully many of my customers either prefer firm nibs or have other interests in fountain pens beyond flex. There is a home for every old pen.