Ive Not seen New parts for Trojan Paul, I think you'll need to find a collector or club who may have old stock, if no one here has a set.
Ive had success with JP before for impossible pistons and rings, if all else fails check em out. Pretty sure they do A fair bit of Villiers as well....like the 7F.
Send them a sample ring and they may be able to match them.
http://www.jp.com.au/JPPistons.html