Hi OP,
Total guinea pig nerd here 🤚🏼
In my experience the best options would be all girls OR one boy and girls.
Two boys are more difficult (it goes without saying you could never have two boys and one girl).
I can't help much with where to get sows but I do have some advice.
As someone else has said, guinea pigs reach teenage years and during this time they will try and establish a hierarchy where one pig will assert dominance over the others. Sometimes the other pig will instantly be submissive but other times they will challenge. They will do various things such as seeing who can get their nose the highest, yawning (to show teeth), rumble strutting and mounting each other.
Many pig owners make the mistake of separating pigs too soon because they think it's bullying/fighting. Of course you CAN get bullying in pigs but you have to be careful to tell the difference between the pigs establishing a hierarchy (which is natural) and bullying/fighting.
After a while it should settle down but the hierarchy can change sometimes. For example one of my pigs grew older and frail and although she was normally the dominant one, her young sister then became the alpha.
Because of this hierarchy it can be much more difficult having two boys. I once had two boys and they went past the point of normal 'asserting dominance' behaviour. There is such a thing in pigs where they 'make a hurricane' basically they fly at each other and fight (looks like they go round in spirals). If this happens you have to separate them as they will fight to the death. This happened to my boys, neither would back down, both wanted to be alpha and I had to separate them , get them both neutered and a female companion each.
My experience of buying vs rescuing pigs is that the pigs I have bought from pets at home have all developed health problems but ones I have rescued have been more resilient health wise. I think it's to do with the inbreeding.
Good luck!!
P.s be very careful with c and c cages and baby pigs. One of mine once got her head stuck. Make sure you have the plastic thing in it so the metal holes are not on ground level