Man you are asking for a bit much now...I had a hard time these last days to even get one of those 16" Shinco double white-line tires...! But no, to what I know nobody is making a 18" in that design...If you find one let me know !
okay I picked up GKM issue #3 I think and inside was a triumph with an 18" dual whitewall tire on the rear. it blew my mind. the tread pattern looked exactly like a firestone 4.00, but it had the dual stripe whitewalls. crazy. cant seem to find that tire anywhere.
Just had a quick look through GKM #3, no Triumphs at all, there is a Shovel, a Knuck ,a 45 and a BSA, all with 16" double whitewall rear tyres, for the record I have never seen an 18" double whitewall.????
Another option would be to get a set of porta walls and paint them with black tire paint 'till only the double white stripe is left. I just checked eBay and there are a gross of listings for porta walls, including 18" and 19" sizes. Again, you would have to touch them up probably at least yearly.
I've used the black tire paint over a white stripe I didn't want, and it worked well. It has to be the right paint (which is hard to find) and like any paintwork, surface prep is 90% of a sucessful job. In my case I used Comet, or something like it, to clean the area I wanted to cover.