> (Does someone sell "decade" dice, which faces say: 10, 20, 300, ..., 90 and 100?)

Yes, they do. I used to use them for this exact purpose.

E.g.

* https://boxcarsandoneeyedjacks.com/product/10-sided-decade-d...

* https://extrememathgames.com/product/10-sided-decade-dice-00...

> (x-1) * 10 + (y-1) + 1

Is that not equivalent to:

> (x-1) * 10 + y

or:

> x * 10 + y - 10