Here's two
http://www.inksupply.com/index.cfm?source=html/canempty.html
http://www.atlascopy.com/cartridges/canon/
The cost for the blanks from either is about double what a filled cartridge
costs from outlets such as tylermartin.com. I've got blanks (still unused
from Atlascopy) and have used compatible cartridges from tylermartin.com.
Those cartridges are now being refilled with bulk ink from
http://www.alotofthings.com That's the least expensive way. Note that
alotofthings uses ink from Formulabs which is a top line supplier of
industrial inks. There are several refill sites offering Formulabs ink, but
http://www.alotofthings.com has the best pricing I've found. They also sell
cartridges, but I don't know if the ink is Formulabs. I'd suggest emailing
them. I know the name is somewhat strange, but they have a highly
interesting web site and my dealings with them have been highly
satisfactory.