The majority of the game’s mods are probably already hosted on the Weight Gaming forums, or have become obsolete - either merged into the main game or lost to various forum purges around the web.
Installing mods is pretty simple, and there’s an older tutorial that covers it well enough, although the formatting’s a bit broken and the mod links are out of date. Generally, all you need to do is merge the contents of the mod archives with your own gamedata folder, although if a mod has a readme it’s probably best to look at that first.
As for mods hosted outside the site, there’s only two I can think of off the top of my head that are worth noting. One is the old Nostordamus mod, still hosted on the Fetish Master dev blog’s mod page. It contains a ton of new content, but because of custom coding on the abdomen organ, most of the content won’t work alongside Juxtaterrestrial’s Belly Mod - or, subsequently, this branch of the game, which adopts a good portion of its abdomen code. The other is Maternal-Reads’ mod, which is another mod with a bunch of new stuff. Unfortunately, it’s also a complete overhaul of the game and repackages everything - the dist folder needs to be deleted before installing it, and it’ll likely cause some minor reversions to game content (locking out choices for what little 0.99-exclusive content there is, and probably some non-fatal issues with things like body fat descriptions).
As for what I’d recommend? There’s a lot out there. I’ve written a couple of them that should be working properly with 0.99, but they’re pretty minor in scope. Jux’s Belly Mod is one of the bigger ones out there, and I’ve been trying to take care not to break compatibility with it. Asimov’s mod has some neat ideas, but not a lot of it is actually complete. ProxyGen is useful for crafting a starting character, although it seems like I might’ve broken something last time I tried it. There are also a couple different mods here that modify or extend Vicky’s cafe questline, but I haven’t had the chance to play with them too much.