BP used to make a pre-mixed fuel especially formulated for the Solex, but usable on any two-stoke. It was nothing special, just gasoline and oil. It was sold through a pump in a few bigger stations, but usually in metal cans like you see in the attached photo of my 3800 at the Bastille Day Rendezvous. There were two types/sizes of cans. A smaller one which was mostly yellow (very rare to find now) for the earlier Solexes and then the green and yellow can like I have, which is two liters. Two liters of fuel in a Solex is 100 miles, so with the original tank full you'll go 50 miles and then with the extra 100 miles in the Solexine can, you've got a range of 150 miles without stopping for gas. There were several different carriers made, some attach to the front forks like mine and some attach to the rear struts. Of course the Solexine is long gone from my can, so I just fill it with pre-mix before any long rides and I'm set.
Some ads for Solexine can be seen at:
The carriers for both the older and newer style Solexine cans and the newer style cans themselves (used) are available from <www.lesolex.com>