I do not carry CO2 cartridges because pumps are much more reliable. On my Luna, I carry a small pump in my jersey back pocket. On my Bike Friday, my pump is held on underneath the tube by skinny bungee cords (but a water bottle mount would work well on a non-folding bike). On my Jamis I ride with panniers (errand bike) so I just carry my pump in the panniers. I have three pumps for three bikes (plus a floor pump).