How to find my LPS number
There are a few sites that make it easy to find the petshop you're looking for, but here are some that I highly recommend:
LPSMERCH.COM - For me, this is the best site when you're collecting; it allows you to have a wishlist and a checklist in the same place and categorizes LPS by generation, mold, gimmick, size, etc.
However, some mention the lack of listings for "Mystery Pets," variants, and prototypes, which is indeed a problem if you really want to see everything, but it works well in many other cases.
Note that the site also lists Pop Mart, My Little Pony, LOL, Monster High, Ever After High, Minecraft, and Nendoroid figures! (And I think they have a category with a ton of other toys that aren't from these major franchises; they really did an immense job!)
PETSHOPSHOOP.COM - A French site that I used a lot when I started, especially to find out the names of the dog and cat breeds of my LPS. Its Skyblog-like appearance might put some people off, but you can't deny its usefulness, as it's the only LPS wiki entirely in French, cocorico!
A collector has been running this site since 2009, according to the legal notices, and all her work almost disappeared forever due to a server change. In the end, we're very happy it's still here; it's a great wiki for the French petshop community!
TOYSISTERS.COM - This site is a little less known in France but is just as good as lpsmerch when it comes to listing toys! As for the LPS section, this site is one of the only ones that lists the very first generation of petshops, produced by Kenner in 1990. The site offers a broad search and some additional information about LPS. You'll also notice that every photo is taken in the same way because Toy Sisters take them themselves!
The only downside is, of course, that as French speakers, purely English-language sites sometimes struggle to please us, but a quick translator should make you love this gigantic wiki of figures and dolls.
Littlest Pet Shop Wiki - Finally, the classic Fandom has its own LPS section, and even if it doesn't serve as a checklist or isn't in French, it holds a special place in the hearts of collectors of more general merchandise. The wiki brings together some pages on accessories, plush toys, DS games, and even old events that allowed obtaining exclusive LPS (e.g., Mail Order).
There's also a page listing the different types of pupils (see here), which, like many pages, allows you to learn a little more about the history of these small figures.
It's worth remembering that, once again, the site is entirely in English, and there's a lot of text, as it focuses more on storytelling than on cataloging.
To conclude, if your search still hasn't yielded results, try taking a look at sites like Aliexpress or minilpsshop to see if your petshop isn't a fake or an original design! As a last resort, it's even recommended to ask collectors; who knows, you might have a rare gem!