There are quite a few online retailers for Inkjet Cartridges. The one I use most lately is 123Inkjet: I looked it up for you, and they currently offer a value pack for 130$. It contains 6 cartridges (4 black and 2 color), which would cost you close t...
Go to the Brother website here (I'm assuming you are in the UK): Fill in the form (not sure why because they don't appear to use the info) and then print off a pre-paid label which you can use to send the empty cartridges back to Brother.
In the preferences, get to it from File/print/printer preferences, and find either black or grayscale printing. Save it as default.