Scroll down there's a small summary of research that has been conducted on an ethnographical basis on yams and their consumption in certain regions of Africa.
http://yalemedicine.yale.edu/ym_su99...tm#bottom_menu
This is from a more naturopathic site for infertility.
http://infertility.health-info.org/f...ovulation.html http://infertility.health-info.org/f...diet-pcos.html
As far as finding them in capsule form, you can find Wild Yams, which is NOT the same as the sweet potato/yams I think you are talking about.
But, I have been told by several naturopathic sources and nutrionist friends that Yams are ONE of the ONLY starchy veggie that is good to eat for PCOS and Insulin Resistancy, because they help control blood sugar and insulin regulation actually.
My uncle who's had diabetes type 2 for 25 yrs, is allowed to eat up to 2 yams/sweet potatoes a day. Not that he does

My mother has an AWESOME recipe for sweet potato/yam bread, I can send it to you if you'd like?
HTH