I'd say it depends as well on what age you are after too.
When I got my first era of GPs (when I was 9yo I got my pig up till I was 22 yo) Ours were Petshop (there were no rescues really) some were home grown efforts because our 2 sows ,erm, weren't.
One was already pg when I bought her.
My DD , just over a year ago, decided she wanted "a pet". She was thinking of hamsters (but I persuaded her to Guinea-Pigdom).
She wanted a boar.
I looked on some local rescue websites and found a local one (it was actually Rabbits but I thought they are bound to have guinea-pigs sent there too).
We reserved two one year olds. Got the accomodation finalised . And here they are.
So if you want adults- go rescue. They might have some young or babies if they've been given a pg sow. But there might be a waiting list if people want a tiny boar to match up.
If you want baby pigs, try a breeder. You'll see the parent hogs. You can get loads of advice.
Just be doubly doubly sure that they sex them correctly (not always 100% though).
Don't go to Pets @ Home- I want the Adoption boars too
"Oh you could live in my Pighouse couldn't you" But my boars wouldn't take kindly.
And Gumtree/Pre-Loved "I want a good loving home for my family guinea. Much loved pet" (I only want £80 for him and his tiny cage). So it's the money, not the home really.
Boars or sows?
My DDs boar is a great big bolshy hunk of love. GP2 is quieter and cuddly.
But they need more space and a bit more understanding when they hit the teens (about 6 months). Mine had to seperate last winter (inside at night) fine in their Pighouse by day.
This year, we put them together but in a quiet room to sleep. And so far, so good.
And dare I say it. They're a bit whiffier. You need to clean them daily very often 