Womentowomen.com is a good site full of info, plus they do a newsletter which is really informative. They are femal OBGYN's and NP's but do recommend things like colon cleansing, etc., which I appreciate.
I first read about natural progesterone in Dr. Christiane Northrup's excellent book "Mother-Daughter Wisdom" although I imagine she has a lot of info in her nest-selling book on menopause. I came across Jon Lee in my later research.
Natural progesterone is cheap and pretty side-effect free.
I forgot to say before, when I was in a twin pregnancy with pretty high progesterone levels (which you want in pregnancy I think!) I did not experience constipation at all.