I was watching a totally unrelated video yesterday and the "teacher" mentioned a lady that has videos you can buy on "how to make a dress form". I went to her website to check it out, and I found that she also sells SEVERAL different sizes of the Flexible Curve (Bendy Curve) that I use for crotch correction.
Her name is Jean Haas. Here are a couple links to her.
You can google her name for more of them. I'm not sure her website is active, but you can go to "Flexible Rulers" and at least see what is available. Maybe you can still order them from her. I didn't try. There is a note about her website changing to a different one -- but the different one didn't have the rulers.
They are QUITE expensive, but maybe they don't flop all over the place like mine does.