Well done
There are a number of issues with getting a guinea pig from a chain pet store. These include that they generally have very poor animal welfare policies, and the animals often come with pre-existing health problems etc.
However, wherever you get a new pig from, you will not know until they meet whether the new pig and the old pig get on. The big advantage from this perspective of using a reputable rescue (in addition to the fact that you’re giving an unwanted animal a home) is that they will often let you bring your pig to “date” several of their single boys and find a pair that works. Even if you cannot find a rescue that offers this “boar dating” service, rescues might also agree to take back a pig that doesn’t work (so you effectively can carry out the ‘dating’ at home).
If you get a new pig from a pet shop and then the two of them do not bond, you’re just going to end up with two single pigs instead of one. This could even happen multiple times…
By the way, if you have not yet read the forum guides on bonding and boar behaviour generally, it would be worth doing that.
One other consideration is that, if your guinea pig is not yet an adult, then he could continue to experience hormone spikes until they reach adult hood. This means that even if you get a pair that works, the bond could conceivably fail again as they age.
I know it probably all seems very complicated. Keep educating yourself, don't rush into anything, and keep coming back here for questions and support as required!