I think it is great that you want to get your piggie a friend.
They really are much happier with companionship.
But I would highly recommend finding a local rescue and working with them.
Many of them offer a dating service so you can take your boy along to meet some potential friends. Plus the whole thing will be managed by experts in guinea pig behavior and has the best chance of success.
Another huge advantage of using an experienced rescue is you will be guaranteed to get a correctly sexed piggie who is healthy.
Many people on here have found that the pet store has sold them a wrongly sexed piggie, and they end up with a mixed sex pair which later results in lots more piggies.
Bonding two guinea pigs can be quite tricky, so having someone on hand to help will make the whole process easier for both you and your boy.
Good luck finding him a friend