It's always worth travelling that extra distance to a rescue, because you know the piggies are going to be well bonded, health-checked, and most definitely not pregnant! If sows, they will have had the 10 week pregnancy watch.
If you can't, then I would be very cautious about getting from P@H. I was told by one of their employees that they only sexed them and split them up when they came in store! No wonder so many people end up finding out their sows pregnant! I've also gone there and seen guinea pigs with bald patches, and one poor fellow had lots of poo stuck to his stomach and had to be taken to the groomers to have it cut out!
If your only choice is to get shop-bought piggies (every piggy needs a home at the end of the day), then I would visit the places multiple times, check out the conditions, see whether they have things like frozen water bottles in this hot weather, plenty of hidies, food, hay, and enrichment, and a large cage, and look for somewhere that will only let guinea pigs go as a pair. P@H only recommends they should be in pairs, but they seem quite happy to let a guinea pig be left on its own after all its cagemates have been sold.