Since we have got members from all over the world, it may help if you please added your country, state/province or UK county to your details, so we can adapt any advice to what is actually available and relevant where you are straight away, and in this case help you with alternatives. Please click on your username on the top bar, then go to personal details and scroll down to location. this makes it visible with every post you make, and saves all of us time and misunderstandings.
I only know of one rescue in each the US, in Canada and in New Zealand that has regularly spayed sows, but none in the UK after the only rescue with a total spaying and neutering policy sadly closed down naearly 4 years ago after a breeder dumped a whole job lot of neglected pregnant sows onto their drive one early sunday morning and left them to pick up the financial and emotional pieces.
Spaying is generally an operation that is only performed for medical reasons as it is much more invasive and expensive than boar neutering.
However, there are many more good standard rescues that offer boar dating under expert supervision at the rescue. Key to any boar bond is mutual liking and character compatibility, and with the right experience and a pool of candidates, boars of all ages can be bonded and re-bonded. It takes on average about 1-3 boars to find 'Mr Right', hence why a trip to a good rescue with mandatory quarantine and vet care as well as guinea pig experience is the best place to do this. If we know your general location, we may be able to give you recommendations as to which rescues offer residential and which offer speed dating. The former generally results in a bond that is every bit as stable as a sow bond and that applies to boars of all ages.
Recommended UK rescues: Guinea Pig Rescue Centre Locator
Rescues in some other countries: Guinea Pig Rescue Centre Locator