You have to buy the entire lever, part number is 45720511A assuming you have the black levers, roughly $120 retail. You might find cheaper ones by Googling that part number/looking on Ebay.
DO NOT buy aftermarket as they are all (as far as I am aware) Chinese knock offs that are made from some metal with a similar consistency to cheese. I've seen the pegs break off these under normal use!
They are a bit fiddly to fit as you have to remove the foot peg support as it bolts to that from the inside, plus the return spring can be a bit of fun to reinstall but not impossible. If you're not mechanically inclined it might be better to take it to your mechanic, it's not a big job time wise (remove muffler, remove foot peg support and rear brake reservoir, remove brake lever - reinstallation is the reverse obviously). One thing you do have to be careful of is the alignment of the rear brake master cylinder and the pushrod that is attached to the brake lever, it's critical to get this right otherwise the rear brake will bind. Sometimes when the bike has been dropped it can move the master cylinder if it hasn't been tightened correctly) the bolts for the cylinder should be fastened using red Loctite, so if you mechanic does the job point this out to him if he's not familiar with Diavel's and check that the brake pedal activates through it's full stroke smoothly with no binding once it's been replaced.