With over 30 million Little Leather Library books produced, these books have found their way through many online marketplaces. If you're looking for a specific title or edition and have already scoured through all the listings on eBay and Etsy, give AbeBooks or Biblio a try - they typically have hundreds of listings for them. Sadly though, many of the listings there are not photographed, so you'd have to go by description to try to ascertain the edition - or message the seller requesting a photo. This is how I found my near mint condition boxed set of real leather LLL books in multiple colors. Be careful trying to just go by description though, as Tom Fox pointed out, many sellers mistakenly think their leatherette books are actually leather.
Aside from those, I've found LLL books and other miniature antique books on Mercari, Facebook Marketplace, and Goodwill Auctions as well.
I thought I would include a section here about some of the ways in which LLL books have aged over the years, and common issues I find. Among the leatherette books, many of the bindings need to be resecured back to the covers, if the cover doesn't detach while opening one up, it may just after a very gentle tug of the bookblock. Another issue with the leatherette books is the brittleness and tearing. For example, a dog ear on the cover can easily lead to the ear tearing apart from the rest of the cover. While the real leather books don't have these issues, the leather can be quite weak and crumbly along the edges. I have one set in which the leather along the spines lost all color, became heavily darkened, very crumbly, and a few have even lost their leather spines entirely.
The pages themselves are typically found secure in their bindings, save for a few loose beginning and end pages. All of the pages though, across all editions, can be rather delicate and could tear easily even by simply gliding your finger across an edge with the slightest pressure.
When cleaning the leatherette covers, do not use alcohol, as it alters the color. In the Redcroft editions, for example, alcohol causes them to turn light brown.