For my 9000 Mk II, I bought Lyson's
empty refillable cartridges (£48) (they reset each time the printer is switched on, so you'd want to check the cartridges every once in a while, or the first you'll know of an empty cartridge is a weird tint on a print) and OctoInkjet's
ink (£37 for a full 8x100ml set), which was highly recommended.
If you go this route, you'll want to buy the "SquEasyFill" lids (come as part of
this kit, which you should buy instead of the ink-only kit as your first set), as the syringe route I went I found to be horribly messy. You'll also
need some gloves unless you like having multicoloured fingers for a few days (a pair come as part of the above kit, but you'll want to buy a box).
Whichever way you go, you'll want to get a calibrated printer profile for your choice of ink/paper combination, as the ink is different to the Canon ink. I used
Pure Profiles, the standard profile for £15.
Yes, the initial purchase is expensive, but consider that a full set of Canon inks costs ~£80 for 100ml total, a set of replacement OctoInkjet inks costs £40 for 800ml total, so if you print plenty, it'll work out far, far better - ~£600 better.
I've no regrets at all. I do do a print every month or so if the printer hasn't been used for a while, just to make sure the head doesn't clog up, but I've not seen any sign of that yet.