Thanks for adding your info! That helps a lot!
In your case, the best and by far safest place for you is Hutches Rescue in Alyth/Perthshire. It is the one rescue in Scotland that we can recommend full-heartedly. They offer piggy dating under expert supervision, so either both girls or just your bereaved girl can decide for themselves who they want to live with. If you can travel there, it is really well worth the distance!
http://www.hutches-boarding-rescue.org.uk/
I have dated several bereaved or difficult sows of mine with both other sows or neutered boars and I have also adopted a pair of younger sows for my soon-to-be-widowed neutered boar Llewelyn (dated himself by a bereaved Dizzy) as I wanted to spare him the pining when his beloved 3 years older "wife" passed away - which did thankfully work!
The key to a successful new bond is mutual linking much less than gender or age. Due to the piggies having the ultimate say when being dated, I have always come off with a happy new relationship, if not always with the piggy I had tentatively reserved! But you fall in love with whoever ends up with you anyway...
If Hutches don't have dateable sows, you could consider discussing the possibility of having an adult boar (over 15 months old and past the teenage hormones in view of your ladies not being the youngest) neutered , so he could live with the girls/your bereaved girl after a 6 weeks post op safety wait; while waiting, he could live alongside, so they know each other through the bars and intros will be easier. Again, if you could let your girls decide who they fancy most, it will go a long way towards a happy relationship. Older sows can be rather particular in who they want to get on in my experience!
We have got a piggy savvy vet locator on the top, and to my knowledge there is a very good one in the Edinburgh area, so in your case, that could be an alternative option for you to research. Having a good vet handy is vital in order to minimise the risk of post complications. A neutered boar would have the advantage that he doesn't upset the existing sow hierarchy, but dominant older sows do not necessarily accept a boar.
https://www.theguineapigforum.co.uk/pages/guinea-pig-vet-locator/
PS: I have tagged another forum mod, who fosters for Hutches rescue.
@lisaali