Try posting wanted adds on the a class and taipan websites. Otherwise AHPC is probably the quickest and easiest. How big are viper boards? You could also try brett at wind rush, he has a few random foil moulds.
Just noticed in a post over at sailing anarchy that the original nacra infusion boards were 9" (230mm) chord which with normal ratios would make them between 20.5 and 23mm thick (smack bang in the middle of your class limits). They will be too long, but it is very easy to cut the top of a board. The advantage with these boards is when they were upgraded the chord reduced (unlike the C2) which means that boats that have been changed to suit the new boards can't use the old ones as a spare without first taking a grinder to the boat so anyone who has upgraded should be willing to let them go at a reasonable price. It would be worth contacting the Brisbane catamaran centre to see if they know of any sitting around and/or posting a wanted ad on the F18 forum.