Collecting any area of philately can be rewarding.
Collecting the
Postal Note stamps on the coupon, or on the entire 3-part form, is the equivalent of the "on cover" aspects of
postal history. It tells a bigger story than a mere soaked stamp.
From experience and
ebay, the mint and used singles are common. The stamps on the 3-part forms are not particularly rare either as many were intentionally saved uncashed from the first day and last day. Searching for a particular town or station may be difficult, but assembling a random set with a variety of cancels would be fairly inexpensive.
With regard to any catalog, I could see a listing of the form variants, but not anything to the detail of a town-list, since there were about 40,000 most offices in that era, 99 denomination combinations, form variants, and likely more than one device used over the period of use = quite a huge permutation of possibilities. Not gonna happen. Besides, the vast majority are not pricey.
For illustration purposes, here is a used set:

Front and back used on the first day:


For the more unusual, here is the Philatelic Agency order form to buy a set:

Comparison of two form versions used in 1945 and 1951.
Even when missing the 3rd right-most portion, note the font differences on the front, circular vs rectangular punches, separation method has paper removed on the 1951 form, and the reverse side had significant text differences. The more you look, the more different they are!


I suspect there are other form variants, but these were two forms I had handy to serve as an example of the collectibility of
Postal Note items. And no, I don't collect them myself.