https://bike.shimano.com/en-NZ/produ.../BB-UN300.html
that's the current gen shimano part and says they have 73mm in 123mm
So you would definately be able to find that part somewhere, eventually. Would your chainring rub on the frame if it was 1.25mm further inboard?
That's the obvious solution. For more waffling see below:

You'll notice that most NDS cups of square taper bottom brackets are not flanged. So while obviously not ideal, you can most square taper 68mm bottom brackets in your 73mm shell, and just screw the NDS cup in 5mm more. That puts your NDS crank in 5mm. Something your body can easily correct especially if using platform pedals or on a beater.

It's also worth noting that cranks will sit differently on tapers depending on wear so it's not even an exact measurement. Like if you had a 127.5mm BB, that would be totally fine. It's mm.