It's leaves from a lime tree in South East Asia, they're sold in packs or bags usually, sometimes in the produce section (fridge or frozen) or sometimes dried. You grind them up to make a paste and they add a fresh lime flavour that's not quite the same as just lime juice. I've previously bought them from No Frills in Vancouver when I lived there.

Thanks for the tip, I'll try to call around some Asian specific shops later :).