i am pro rescue as well but did buy a pet shop piggy as my first. He was sexed properly despite being extremely young, perfectly healthy, easily tamed, and all round a great little pig except for being picky about his veg! Whatever you decide to do i'm happy to hear you're giving an animal a loving home
I'm in canada and guinea pigs in stores around here range from 20-25 dollars whereas adoption from the ONLY GP rescue around is $30. I'm starting to think that the UK may have better rescues and vets, but here in Canada, in my area specifically you NEVER hear of pregnant females being sold whether its guinea pigs hamsters gerbils etc, in fact for guinea pigs the shops here only sell 1 sex. So if you want a specific sex you have to call multiple locations to see which one is currently hosting that gender of piggies. I also purchased a dwarf hamster from the local shop (no shelters here seem to house hamsters and I didn't know to check Kijiji back then) and again he was sexed correctly, easy to tame, and in perfect health. The petstores here are guilty however of too small cages and unhealthy treat selling but other than that, in my experience and all of my friends who also have small pets, there has never been an issue. Not even mites or flees or fungal issues from any of the guinea pigs sold there