I've had a 158 machine and found most of the monogrammers that work with it are either flat bad or open arm. The cams were the same for both of them. These units are not rare and most often came with a full set of cams so those aren't rare either. The are long rectangle shaped cams.
However there is also the Greist version that was sold under several names, Sears being one of them. Most were sold under Singer, and those are the spendy ones. The cams look similar to the Kenmore ones, but are not interchangeable. Greist also sold this unit under it's own name, and I own a Greist, Sears and Singer, and they are all the same. These units were sold with a voucher for three cams, and though full sets could also be bought, they were uncommon and that is why trying to get a full set is expensive.
I got the full set by watching ebay for monogramers. it was a Greist set (not marked Singer on the box) and had a typed paper with instructions on how to use it with a Bernina machine, so the seller advertised them as Bernina monogrammer cams, and that didn't garner much interest.