I'm planning on doing some conversions soon (Namely female versions of 40k models. Some will be based on famous memes such as Lolicron, Khrone, Nurgle, and Tzeentch Daemonnettes, Hive Mother, some female Space Marines, and Heavy-Chan.) and was wondering where could I find some desirable female heads?

I can't use the ones that GW uses, mainly because they're.... well.... Hideous. Sure, in the 41st millennium, cute or beautiful is rare.

Or should I start making them from scratch from Green Stuff?

There are some, but they are in pretty short supply:

These are quite good, i have used them myself on some IG models.
http://www.shop.microartstudio.com/wome ... p-360.html

These are nice, they are a bit smaller then the ones above and have no hair.
http://www.hfminis.co.uk/shop?category= ... converting

Also although you say you dislike GW ones, the New Dark Eldar female heads are quite nice.

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.

This may not be something I'd order just to get the heads, but there's this nice kit from Titan-forge:
http://titan-forge.com/sci-fi_miniature ... ssin_squad
I'd consider putting it toghether with the more... awkward heads, as they match the general theme... And the remaining ones are quite nice.

Wish they'd start selling the heads alone. Still, their FAQ has this rather promising point:

Q4. Can I order parts of your kits separately?
A4. Not all of them, but sometimes yes. The only condition is that you have to order no less than 10 parts of one kind.

Thanks guys, I think I'll try to order from Firah's link.

But I suppose I could look into the Dark Eldar's female heads......

:O would be awesome if those heads did come separate, they are lovely