I wouldn't worry about the fly strike thing to be honest - as long as the cage and the guinea pigs are kept clean then you have nothing to worry about. Flies go in our shed especially when it's getting near to the time for the hutches to be cleaned out but none of my piggies have ever had flystrike or my rabbits who live outside in hutches.

Have you tried your local RSPCA, they might have some guinea pigs?

I have two indoor piggies that live seperately as they are male and female. The female is bossy and doesn't get on with other piggies so I keep her on her own but she gets plenty of attention and loves living next to the fridge.
She's in a Nero 3 but for two piggies you will need a Nero 4.