I am looking for a producer that might be interested in a reality TV show and accept a pitch.
Then reach out to TV producers, I don't think you'll find them hanging out here...
IMDb Pro, Mandy, and personal networking are your best bets. Also worth mentioning, I don't know ANY media companies BUYING reality TV shows... the entire reason these things exist is because they are incredibly cheap and easy (relatively speaking) to slap together, and bring in big bucks for the networks. These types of programs are almost always packaged in-house. Not to mention, there is no shortage of ideas, and it sounds like all you have to pitch is an idea (everyone has an idea in entertainment, everyone has a script.... these are the cliches that constantly ring true).
So not to end this answer on a downer, I'd buy an IMDb Pro subscription, if you're serious about this, and start pitching producers that have worked on similar projects, but... realistically, I wouldn't expect to get very far.
Higher value projects that producers are more open to hearing about from outsiders are original narrative feature scripts, or original series (you'd need to have at least a pilot and bible written out, and potentially a full season).
- I worked in film/TV for several years in L.A. and Seattle