Sorry for the long delay in my reply. I have a right side tank from a '99 Rallye that I will install to my '03 Adventure, pursuant to the work done on this site
I guess there is no reason to be selfish about my source for the side tanks. Let this all go out to everyone then and always speak fondly of Slappy. Both right and left side tanks and other associated parts can be found right here in the US. There is only one of the parts listed that is not available and it is not one of great consequence. See this site
Click on parts catalog (upper left). Go to 1999. Go to 1999 Rallye 660 Div (088172). Go to fuel tank; seat; additional fuel tanks rallye 99. Choose your parts. That is how I got mine just a couple of months ago. Sorry about the length of that mess but it is surely worth it. Enjoy.