There are some spam/bot-filled ones out there that claim to be anal dating sites, but I wouldn’t expect much from them. Dating based entirely around a specific sexual concept isn’t generally going to go all that well. Rather, look for someone you like in a general sense and who is sexually open-minded, and discuss becoming anal only with her or just explore anal together without any commitments at first, make it really pleasurable for her and make her feel sexy about it, and if she likes it, she’ll probably want to do more, and from there you can ease naturally towards anal only together.
The majority of anal only relationships evolve naturally in that manner. Others have luck identifying early on as being anal only and wanting an anal only relationship, with varying levels of success. Being up front works great for some, and getting to know someone a bit more first works better for others.