If you're fully slalom racing, you may like to hotrod the ST115 with some full race/slalom fins, which work quite well with ST when used in "slalom" mode (c/w SuperX or freeride mode/s). The choice of fin style also affects the respective sizing/s (as the stock 33 (2006) ) is a more mixed freerace/freeride 33, which for your size will be good for medium or light 6.2m / typical 6.7m / well powered 7.5m conditions.
If you take a more vertical race fin, for that range the size will be more like 34 or even 36cm, for lighter 7.5 consider race 36-38cm - and for 5.4m onshore freeride, consider something lower aspect but still fast like the stock freeride style but sized 28(30max) cm.
Yes, you should be able to buy Drake original fins in Germany, if any problems please contact German National Distributor :
APM Marketing GmbH
Ph : 49 89 89511170
Fax : 49 89 89511172
Herr Stadler und Herr Mencke sind sehr Spa?kerle
Cheers ~ Ian