eBay is my best source, but I just pick up disposable paperbacks. Maybe you could use a saved search and get a notification when a listing appears. Are you assembling a collection of rare running books? There are only two copies of that Shapiro book on eBay and the cheaper one is $83.
Yeah, might have to just put it as a notification and maybe reach out to the local library and some Facebook groups (crazy that that might be the best bet). What if I told you that there was, finally, a 2nd edition of Ultramathon published and you can pick it up for less than $20 (less tax and shipping) from the publisher? https://www.echopointbooks.com/sports-recreation/ultramarathon UPDATE: I found one of the books I'm looking for, Four Million Footsteps, through a one-person publishing company out of the UK. How fitting. The single 1st edition copy available on Amazon is $250, so you should consider the $80 for a 1st edition of Ultramarathon to be a bargain!
Update for you and wasoxygen - found that book, and the white whale, My Run Across the United States last night!!! Took 3.5 months...
Interesting, one eBay listing says the publisher of the Shapiro book was Bantam Books, that venerable house of pulpy paperbacks. The new edition is printed by Echo Point, an aptly named print-on-demand service for out of print titles. With shipping, the paperback is 50ยข cheaper than on Amazon, and the up-to-10-day lead time to run off a copy gives me time to make progress in another eBay book.