Definitely try specialized used book sites like AbeBooks or Biblio, they often have small publishers and bookstores that sell their old stock, including Thilliez's early works! It's true that Vinted and Le Bon Coin can work, but you really have to get lucky and scroll for hours. Physical bookstores are cool if you've got a good one near you, but honestly calling around everywhere can take forever, whereas on specialized sites you can see right away what's available and at what price. Also check out small French online booksellers that specialize in thrillers and crime fiction, they sometimes keep back-lists permanently in stock!