Well it mostly depends on what style you want to go for. If you want to mostly keep the
GW style there isn't that much I know of. There are a few models here and there. (for example
http://www.svarogminiatures.com/) But nothing in a large enough number to make an army that doesn't look like a clone army.
I heard rumors that Raging heroes might do a sister-esque army. But it won't be too soon. And even their current toughest girls of the galaxy models contain some that might work as sister base model.
Then there are probably a lot of different models from other games that could work, like those:
http://shop.prodosgames.com/en/brotherhood-support/54-valkyries.html
Reaper Miniatures also have a lot of female knights and some sci-fi minis that can work.
And there are a lot of others out there, depending on what style you want to go for and how many time and money you can invest.
What I've seen in the conversion area is sisters made from space marine scouts and a lot of other
GW bits, Alternatively there are the female eldar and dark eldar models that can work in combination with space marine bitz and dark/wood elf pieces.
Like I said. It comes down to what style you want your alternative sisters to be.
Personally I like the
GW models design but hate the limited conversion possibilities of metal models ( and the price is offputting as well). So I started sculpting my own and plan on casting them. (Link in my sig)