I worked in the precision potentiometer industry in the 70s and back then there were dozens of manufacturers that would be glad to sell you standard, semi-custom and full custom resistive track elements in 100 piece volumes. As far as I know all of the companies fell on hard times due to potentiometers being the fail point for so many systems because of their limited rotational life. Only the large companies that specialized in larger volume lower cost potentiometers survived the 1980’s intact (e.g. Bourns, Spectrol, Phier). They along with Honeywell bought up all of the other companies that offered low volume production. To survive the industry ended up becoming exclusively traditional electronic component suppliers with standard product lines and 1000+ piece minimums for semi-custom and full custom orders. I suspect if you could search long enough globally you could find a manufacturer who would do full custom for less quantity. However you’d likely get sticker shock on the price. Even in the 70’s a 100 piece order of a semi custom low to mid accuracy decent life track element had a per part price of $25.00 or more.
Given the life span issues of potentiometers used as sensors (every tiny movement counts against a relatively short total life) nearly all sensor applications moved to digital encoders (optical, magnetic, or mechanical) or linear hall effect for the sensing technology. For ultra-miniature size, a raw die linear hall sensor with a very tiny neodymium magnet would be smaller than any resistive track system and have theoretically infinite rotational life due to being non-contact. The ordinary small standard component surface mount hall sensor packages should be able to compete with resistive elements for ordinary small space applications.