105 8 Speed spares.... anyone know?

[Pupp]

...whether STI brake/shifter levers are rebuildable and, if so, are spares still available. My right one keeps 'missing' changes down the cassette (ie with the little lever) and I think the ratchet (I presume there is some sort of ratchet mechanism) might have shed a tooth or suchlike...

Thanks for any pointers