there are actually some other parts that may be needed to repair the horn properly, just replacing the plunger and spring may not be enough to make it last.
As adams1 said you will need:
9135142600 - plunger
9135405600 - spring, earth

You may also need:
9135408900 - carbon washer
9135402400 - plate, contact
9125501800 - spring, horn
9135403100 - plate, set
9135402500 - packing

Now although you might can get by just replacing the plunger and earth spring i have had many of these where the contact plates are worn out and even after cleaning them up they still did not want to work properly, usually you can get by not replacing the plates. The spring, horn may be ok as well, just use the old one unless it is corroded. The packing is just the rubber seal around the button area.
Now... the most important part in these other parts i've listed is this carbon washer, mcfa came out with a hpk service gram recommending to add this carbon washer under the steer wheel, between the steer wheel and top contact plate ring on the steer column. This carbon washer is the new conductive surface for the plunger to ride on instead of the brass ring on top of the steering column, there is a considerable reduction on wear on the plunger when installing this carbon washer so you will not be replacing it every few months like what was happening before then.
So i would at least recommend getting the carbon washer kit also when you replace the plunger and earth spring.