only tyres that are listed as xx-584 (in ISO size designation) will fit 650B rims. 584mm is the bead seat diameter of the rim.
When you say '
26" wheels ' you presumably mean -559mm size; this is commonly meant but is potentially a very misleading way of referring to a wheel. 559mm is just one of five or six rims that are designed to yield wheels that are 26" rolling diameter, including 584mm, i.e.
650B is a 26" size. Others include rims that are 571mm, 590mm, 597mm. Each of these rims is intended to take a different section tyre and yield a wheel that is 26" in overall (rolling) diameter. A machine fitted with
any of these would correctly be described as 'a bike with 26" wheels'. Tyres obviously do not interchange between these rims.
Remember that (although there are some tourists using them) the current fad for 650B rims (as evidenced by the fact that most of the rims you can buy are lightweight 32h ones meant for disc brakes) is mainly being driven by the MTB market, with folks (inevitably) finding that they want some kind of road tyre to fit an MTB with ' 27.5" wheels ' (another daft way of referring to 26 x 1-1/2" rims....
). Most riders of MTBs are not going to be looking for a road tyre as narrow as 32mm so the tyre you seek will probably remain a somewhat niche item. 38mm tyres will likely always be the easiest width to find in 584, because that is what the standard width is for that rim size.
If you don't like the tyres that fit 584 rims you could get a set of wheels with a different rim type in your frame. If the stays and forks are wide enough you could get 559mm rims and use a tyre of about 50mm section (which will be about 25-3/4" overall diameter). If you put 622mm (700C) rims in that frame a skinny (say 25-622) tyre will give a wheel that is about 26-1/2" rolling diameter and that may fit too.
BTW the closest standard size tyre/wheel (with some availability rather than none) to fit under your mudguards is 26x1-3/8" (35-590) . This was a standard size fitted to millions of roadsters made by Raleigh etc and not that many years ago it was one of the few tyre sizes that could be found World-wide. However that tyre/rim size is another size that is slowly dying; it is still fitted to a few new bikes but not many. I don't think anyone makes decent double-walled rims (I have only seen single walled, 36 drilling in recent years) and choice of tyres is limited. However you
can buy Schwalbe marathons in that size, so if that is the sort of tyre that you want to use, it isn't such a bad choice. [It is also a common wheelchair rim size, so is unlikely to go extinct completely.]
So there are a few choices. I expect you will find a -584 tyre that will suit you alright, but if you don't there may be another way with that frameset, (provided it doesn't have cantilever brake bosses, which would make changing rim size problematic).
BTW I have a folding bike that uses 559 wheels (no choice, cantilever brakes fitted) and it has ~32mm tyres on it. The rolling diameter of the wheels is ~24-1/2" or so, so the bike packs down small. I expect to be able to buy good tyres in that (niche) size for at least five or ten years to come, maybe longer. As it happens the frame will accept tyres up to 50mm quite happily so I am unlikely to get stuck even if (as pessimists would have it) in time, tyres for the 559mm rim become less plentiful; I will always be able to buy
something.....
cheers
).
