RE: iSonic 105 Fin Suggestion for lightweight sailor
The shorter versions of the Drake Pro (30, 32cm) should be available in USA via dealer order from Trident Sports without too much trouble.
Other options will be influenced by what is available (readily) locally at your location. GSport.com in HoodRiver have the SR6b, which is a good fast fin family ~ but it's a different nature to the DrakePro - and maybe not as ideal for lighter riders in your conditions once conditions start getting rough and the rider/board/fin start getting thrown around.
Sidebar : [it could be that a slightly more freeride fin- rather than pure race- could be an ideal compromise for a lighter guy struggling for fun/control in maxed out choppy iS conditions. There is serious technical merit for this, as super hi speed fins have fine foils, which don't develop real lift until they are moving really fast through the water, and usually in a narrow (controlled) angle of attack range. In rough, choppy and bouncy conditions, the actual board speed may feel fast but not be that quick, plus your fin's angle of attack will be all over the place (as the board is flying all over) - and the world's fastest fin won't be the best fin for those fast "4WD" conditions. Here something slightly "slower" (but with more "grip" earlier in the speed range, and a more relaxed foil [+angle of attack] will often be a better choice, and potentially even "faster" for sport riders in those control conditions. (eg; Drake FreerideFlow (new 2007) or GSPORT : Freeride) ]
I'd really recommend trying to borrow (or test) some smaller fins in your conditions before you buy; hopefully you can get hold of one or two options in the 30-32cm TT freeride/slalom size (not too rare !) - at least this gives you a real world benchmark ( for you, in your conditions) before you lay your $ down.
Cheers ~ Ian.