I think there could well be other ladies in your small town that know very well that you're interested in women. We often do 'know'.
Perhaps your friend is straight, or perhaps she isn't but would be horrified at the thought of a married woman just using her for sexual gratification. That's how I'd feel. You're in a committed relationship, marriage, and even though you say your husband is ok with your desires that's because the two of you are speaking only of a sexual aspect. Many straight men like the image of 'girl on girl action'. Many Lesbian/Bi/Bi-curious women though would be really put off and potentially HURT by your attitude. I would be really upset if a woman I was interested in only wanted to 'scratch a sexual itch' and fulfill a fantasy for herself and her husband - I want to have an intimate, loving, monogamous relationship, with a sexual component, with my girlfriend NOT to be used. Because you already have a friendship with this woman, your relationship with her already has an emotional connection - Things could get VERY messy!
There are people out there who only want a casual sexual encounter like yourself, but I would suggest a personal ad or internet site for meeting like-minded people, without complications. Don't ruin a friendship by complicating it by making sexual advances but expecting the emotional dynamic of the friendship to remain the same - You are more likely just to lose a friend or end up feeling weird and awkward around each other regardless of wether she wants to sleep with you or not.
HTH!
P.S. I know this is an old post and 'you' (ie the original poster) have likely moved well on with your life since then, but this issue comes up so frequently in 'our' community and online that I thought putting up a reply anyway might be helpful to those who stumble across this while looking for similar advice themselves.