Best to get the replacements off the net.
I paid just over a fiver for a black and a colour cartridge for my Canon i455, delivered and including VAT. The cost of cartridges for my old Epson Stylus 480 is a bit over £4 for a black & a colour, that's less than half the price of an epson black cartridge.
I've never had any of the quality issues these cheap cartridges supposedly have.