With a few minor exceptions (like pins and shackles), all parts like, rigging, trampoline, sails, mast, rudders, etc. must be OEM (in your case Hobie). Find a copy of your class rules - it will all be specified in there.
Regarding the rudders, the least expensive rudders are molded from plastic while the 'racing' versions are a much stiffer composite layup. The plastic ones will tend to flex a lot if you are in choppy or confused conditions. Yes, they are all the same from the Wave to the 18 - only the hole locations change slightly and I believe you drill those out once you receive new blades. If you are in the market for a replacement, keep an eye on E-bay. They're on there frequently.