if you don't already have an ed please don't try to join any :/
will probs make you get an ed at some point, plus pro ana doesn't exactly mean being for eating disorders anyways usually. it's more like being pro letting you choose to recover or not at your own pace & try to live a good life even with an ed. people who are like litterally "eds are so quirky/i want to be anorexic/ it's a lifestyle" are super looked down on in those places usually though js.
anyways, you got the classic mpa/myproana forum (the biggest site for this stuff) and newer similar ones like eating disorder central (smaller version of mpa, not many people on there) & mypancakeaddiction (also a small forum). you also got other forums that are more obscure, don't really know that many though (cos there's lots apparently) except skinnygossip, cerulean butterfly & projectshapeshift. you can find more if you look on mpa.