Aside from your original horses, there are a few divines that give foundations (Kaktos, Ran and Hipparion) and then there are Prodigy horses which used to be Champion Seeds, which I'm pretty sure are also foundies (Yellow Dwarf gives those).
Otherwise you can occasionally buy foundations/Prodigy horses in money packs when they come around.

also when the prize in a promo is a horse. like when a new breed is released the horse is a foundation

Ah ok. Thanks.