Or you could just use regular marine models with helmets. Marines without helmets are pretty silly, anyway, and there's just no reason at all to presume that armor for female marines would have breast bulges.
Realistically, chest plates are bulged anyway, to help shed blows to the sides, and Space Marine chest plates are pretty obviously bulged--there's really no way that there isn't enough room in there for a pair of breasts.

I could put my army on the field and say that every single one of them was female, and, modeling wise, it would look exactly right. There's just no need for feminine features to denote femininity on models in powered armor.

That being said, I can understand wanting some better-looking female heads for those of you who like the cinematic appeal of the occasional helmet-less marine. I still wouldn't use breast bulges, though. Those really just end up looking silly.