I own a 26er myself .... why not ride it until one of us expires? If you are worried, buy a stock of tires and tubes, but store them in as close to an airless, dark environment as possible. Otherwise ... If someday I need a tire and there simply are no more ... i will deal with that. Not thinking that day will happen soon.

Particularly those old-school MTBs .... serious tanks. Good for everything except road racing.

As for bar size, I bet there are any number of replacement stems .... if needed. If it is a 1" threaded headset .... I bought one a few years back, a Ritchey I think, for $10.

Another option .... start buying up all the junk/trash/Craigslist giveaway 26s people have had in their garages for years and cannibalize them.