1/48 scale figures are a good source for alternative heads. Female figures are pretty rare, but available. Soviet Airforce and RAF ground crew kits usually have one female head, and male heads usable for other projects. There's also resin kits like this http://www.hlj.com/product/KAD48-006
, a female japanese self defence force figure in resin.
A better source of heads is 25-28 mm metal figures. I use Eureka miniatures. http://www.eurekamin.com.au/
They have a variety of decent figures of various sorts at reasonable prices, but suppliers more local to you may be a better bet.
I've also found a plastic Sisters of Battle head in the Immolator kit, but that's one expensive way to stock up on heads.
Looking at the link Firah provided to micro arts studio, I think that'll be your most economical choice.
Have fun, and post some pics when you've found what you want.