It's not that rare. Certainly not unheard of. There's plenty of sites and groups about it on the web, though few are done tastefully. I've toyed with the idea of this fetish a bit myself. I've been turned on by pictures of women shitting, and even videos. And I am interested in the anal side of sex to a certain degree. But not enough for it to be a major fantasy or fixation for me.
If this is a major issue for you--as in a fixation to the point that it's the only thing that can get you aroused anymore--I recommend seeing a professional for some help about it. But if it's just a sort of small fetish on the side, I wouldn't worry about it. Curiosity and interest in something considered taboo is normal, and this definitely qualifies in my opinion. It sounds to me like it might be closer to a fixation for you though. Have you ever thought about why you have this fetish, or what the nature of it might be?
As the previous poster stated, is going to be pretty hard to find a woman who shares this fetish, but they are out there. Just approach it very cautiously if you do choose to bring it up to a partner, and if you do so with one who's not into it, don't force the issue. It would be your privelege to explore this fetish with somebody, not a right by any means, so never expect a partner to cater to it unless they're into it as well. But you probably know that already. If you really do want to explore this fetish though, I recommend maybe finding a decent site or group dedicated to this fetish and maybe meet somebody that way. I'll see what I can find and link you to if I can.
Best of luck to you.