>>241278
Depending on what level of quality you're willing to work with and/or how much editing you're wiling to do, there's a few options for getting premade models:
Most obvious choice is SmutBase, basically a big repository for blender porn models and plugins. Most if not all of them come with nude bits available under the clothing or as the only version, and they're typically pretty decently high-poly with good textures, so modifying them to be fat should be pretty easy if you know what you're doing. They're usually pre-rigged as well, both with standard bones usable by game engines and with whatever fancy unique animation rigging blender uses specifically for rendering/animating, don't really understand the latter but I'm working on learning it along with other newer elements since my knowledge is stuck around version 2.98.
Second best option I know of would be its sister site, SFMLab. Much more SFW, but if you're already modifying them to be morbidly-obese, adding nipples and vaginas to them if you decide to isn't a huge leap, especially since SB mentioned above already has premades of both you can stitch onto models to add them easily. Has a lot more model variety thanks to not being as narrowly-focused on porn, so it might be better for finding more niche content or just a better variety in general. Should still be of similar quality in terms of polycount, meshes, rigging and texturing.
Next up is Models Resource, which is a big-ass dump of ripped models from a wide variety of console and PC games, of varying quality and complexity. Unless you're downloading relatively-recent game models like current-gen PC and console ones, or staying within the polycount and style constraints of something much older like N64/PS1 on purpose, you're probably going to have do a LOT of modification on these to make them both fat and decent-looking. Another consideration is that MR is by nature limited to only copyrighted characters, even in the customs sections. Also, a lot of them come unrigged, so I recommend either being familiar with rigging models yourself or using something automated like mixamo/accurig to do it for you easily.
Lastly that I know of, you can trawl through deviantart, sketchfab, opengameart, and various other decently well-known model repositories to try and get models to tinker with. No guarantees you'll find anything good this way, but you never know, sometimes you'll get lucky and find a diamond in the rough. I've seen some fantastic BBW models on sketchfab for example, but sadly they were set to not be downloadable and only viewable, for some reason.