Yeah, Ive keep an old motorcyle tube laying around for the past 30yrs. just to have some when I need it. Bicycles still use them too, and it is much thinner.
O-rings are available at any hardware store (plumbing as well as 'bins') and auto parts stores, so they aint far away or very expensive. Your local auto repair shop may have some 'used ones' that they dont dare use for oil control in a rebuild but could bear to throw away yet. Ive even found them laying in the gutter (hey, you can laugh, but they go in the utility bin!), but dont depend on shopping there.........(geesh, youd think I was a 'Depression Baby')