Have you read “Open Embrace” by Sam and Bethany Torode? It’s one of the best books I’ve read on nfp. They are a protestant couple who felt instinctively that birth control was wrong and began looking at what Scripture had to say, and really became passionate about nfp in practice and its morality. I highly recommend it.
To learn the method and get support, definitely check out the Couple to Couple League to take either a course or home-study class. Though it’s a Catholic organization, I know there are protestants who take the classes as well.
“Taking Control of Your Fertility” by Toni Weschler is a good book for learning about fertility signs and the nfp method. It’s not a Catholic or Christian book though, so it will simply teach you the basic physiological components of nfp. It’s very in-depth and a good resource though. “The New, No-Pill, No-Risk Birth Control” by Nona Aguilar is a favorite of mine. It goes through the method of nfp and also discusses the emotional and spiritual aspects of it more (but it’s not a specifically Catholic book). It has real-life comments from couples who’ve used nfp that were a great assurance and inspiration to me before we were married and before we had used nfp.
Some friends of ours are Protestant but just recently started using nfp. They are hard to find, but they’re out there! Best wishes in finding support…feel free to come to this board if you need any support from fellow nfp’ers!