Thistle
Cavies is a fantastic rescue. The lady who runs it knows everything there is to know about guineas and takes such great care of them. If they are too far away you could also try the Scottish SPCA.
Animal Welfare Centres and
Rehoming category-list
Try phoning your nearest SSPCA centres, because they often get piggies in, but they might not necessarily be up on the website.
Local Adverts and newspapers would be an option, but yes, beware breeders. If someone is advertising a piggy for adoption with a price, then I would beware in case they are a breeder. I've seen a lot of ads in newspapers where people are trying to recoup the costs of their hutch by trying to sell you that for a reduced price, with pig 'free' whilst asking you to adopt their piggy. I think they should be ashamed and concentrate on finding a good home, but there you are. You'll have to use your own common sense and be prepared that the piggy could have fungal, mites, or other medical problems through poor care. SSPCA piggies have been chacked over by a vet, and Thistle
Cavies ones are fully healthy before they are rehomed. With newspaper ads, if you don't get for free the piggy and everything the seller has to take care of their piggy, ie, cage, food, bedding, water bottle bowl etc. then I would be suspicious as to why these items were missing and why they wanted money for you doing them a favour.
You could try advertising that you are willing to adopt a male piggy.
Let us know how you get on. There is another rescue that I know of, I think it's called 'scotrat' or something. Try a google search for scottish guinea pig and rabbit rescues if you are stuck, or try searching through rescue sites outside scotland for lists of rescues nationwide.
Good luck!