For 4-strokes, there are sets of shims for 250cc and under and there are sets for above 250's (I'm talking diameter). So, the size for your Z are the same as any 450, 500, 510, etc. As far as what thickness you'll need, it depends on what gap you measure and what it'll take to get it back into spec. Search for valve shimming how-to and it should have a link to a valve shim calculator. Just take your measurements and enter them into the calculator along with the shim sizes you currently have, and it'll tell you what you need. Most local mechanics will let you trade shims with them for free.
94mm bbk, +4 stroker, Hot Cams, Vertex piston, FMF Power Bomb header and Power Core can, KW black diamond valves/dual springs, FCR39, K&N, Elka long-travel LT fronts and rear, Houser +3, +3/4 LT A-arms, American Star pro xs chromoly tie rods, Dyna non-programable, Tusk +3 axle.