Many hardware stores have a selection of o-rings.
On mine the hose to the external tank started leaking where it screws into the Stay A While.
The sealing surface of the Stay A While got buggered up somehow and I couldn't find the fitting anywhere including the company that sells the Stay A While so I put a small O-ring in it and that has worked great for two or three years.
I know this isn't where your problem is but I thought it might help someone reading this.
Clay (WA5NMR), Lee (Wife), Katie & Kelli (cats) Salli (dog).
Fixed domicile after 1 year of snowbirding and eleven years Full Timing in a 2004 Winnebago Sightseer 35N, Workhorse chassis, Honda Accord toad