You're meant to wash guinea pigs about once every 6 weeks, to keep their coat and skin in good condition. We adopted our piggies just before Christmas, and they were starting to whiff a bit, so we bought some stuff from the place recommended by the rescue centre we got them from.
The shampoo and pre-wash treatment are both from www.gorgeousguineas.com. They're very gentle, and smell lovely too. I don't think Candace was too impressed, but she did smell lovely afterwards, and has forgiven us after the liberal bribery of much cucumber and sweet pepper.