What are some good supplements to feed to pregnant mares?

I just started working at a local ranch and the owner has asked me to find some good supplements for her pregnant mares. The mares are no more than two months pregnant. I been doing some of my own research but I figured that the only way to really know if a supplement is good is to ask the people who use it. So if you are someone who has cared for pregnant mares and knows of some good supplements for them that would be great. Also, if you have any suggestions for what to feed them after they have their foals (this is a few months away though) that would also be helpful. Links to websites or stores where I can order would be very appreciated. Thanks!