Personally, I've never bought a guinea pig, I've always rescued and there's usually a minimum donation for this, it's not based on gender.
I've seen guinea pigs in passing when I've been forced to go in pet shops (have a quick check, make sure they're ok!) and have never seen females for sale. My assumption is that these care kept for more commercial breeding mallethead
My suspicions were pretty much confirmed when I started volunteering in the small animal unit at the RSPCA and asked if they had ever had any female pigs, as all they ever seemed to have was male. They did have a couple of females brought in, all heavily pregnant mallethead
You generally find however, if you get a dog from a breeder, the females are normally more expensive than the males. Again, I've never bought a dog, but just going by what I've seen!