Salty Dag Marine supplies parts for Hobies and other Beach Cats. They have a 1-Peice Bias Cut Trampoline for the Hobie 18/18SX that they retail for $299.70.
http://www.saltydogmarine.com/
I've also seen new trampolines on e-bay ranging from just over $100 to $300. One Ebayer (ebay seller name: hobiemill_llc) sells hobie tramps (Pennsylvania Amish make). He currently has two on auction on ebay motors. (Item number 160053493315 and 160054289937)
http://motors.search.ebay.com/hobie-tramp_W0QQfromZR40
The "Hobie-Bob" rotating mast float lists for $186.00 in the 2006-2007 accessories catalog. The "Mama Bob" lists for $120. I've adapted a "Baby Bob" to the my H-18 mast tip (non-comp tip) using some hand fashioned aluminum brackets. The Baby Bob lists at $96.00. It is smaller (Displaces 32 pounds of water vs 60 pounds for the Hobie Bob). And does not rotate, but it does supply a reasonable amount of buoyancy at the mast tip. All three are available on the Catamaran Sailor web store.
http://store.catsailor.com/tek9.asp?pg=products&grp=146
For both items I’d guess that shipping to Argentina may be expensive.
Good Luck.
Fair winds and following seas