If you don't mind shopping from overseas check out Punyus. It's a Japanese women plus size clothing brand. It looks like they now do international shipping.
There is a size chart on the item's page. Up to a size 6. You probably already know Japanese sizes run small over there so it is best to size up. I bought a size 3 dress from there which I think is xxl. My usual American size is xl. Here is their detailed size chart.
punyus.jp/guide/size/I would recommend for your girlfriend to measure herself using a measuring tape that way she can compare her measurements to the ones online. Some Asian manufacturers measure flat which I think she needs to double her measurements if she ever does the latter method.
Punyus also has this feature where you can input your measurements and it will estimate what size you are. I can't find it at the moment. I think it appeared when I made an account years ago.
Edit: As someone who has been doing research on plus size fashion sites. I do have lists of miscellaneous clothing websites scattered on my Google Drive and notepad app. Even some Etsy shops. If you like, I can eventually share these sites.