If you are looking to sell, it is probably something collectors would buy; not people who want to shoot it.
If you are looking to buy, I don't think anyone makes it anymore (not even brass for reloading).
This thread explains quite a bit about it:
ht tp://www.marlinowners.com/forum/32-40-38-55-375-other-great-non-standard-calibers/80689-marlin-25-36-questions.html
If you want to load for it according to the guys in that thread you should look for 25-35 Winchester brass to reform into 25-36 Marlin. The cases will be a tad short but will still work. Since the 25-36 Win is based on the 30-30 I suppose reforming 30-30 brass could also work; they all use the same case head.