Soldato
I can only find them on ebay abroad. There are some new seals but I'm not sure I'd want to mess around with that then find it doesn't work, might be worth a go I suppose.
http://www.ebay.co.uk/itm/122312507708?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT Is this the kind of thing that would fix it?
I honestly wouldn't know, I've never looked into rebuilding a clutch/brake master cylinder before.
Here's some used ones:
http://www.ebay.co.uk/itm/Aprilia-RSV-1000-Millie-2000-Clutch-Master-Cylinder-Lever-Supersport-/272399209321?hash=item3f6c421369:g:2fIAAOSwLnBX8Ylo
http://www.ebay.co.uk/itm/APRILIA-RSV-1000-1998-2003-MILLE-CLUTCH-MASTER-CYLINDER-USED-MOTORCYCLE-PARTS-/191268994852?hash=item2c88853f24:g:-4AAAOSwDk5T2Nqv