https://heritagetile.com/subway-mosaics-design-tools/

Yay for saving the vintage bathrooms! Yours looks absolutely gorgeous.

B&W Tile is much beloved by old house folks, but there are a bunch of other places too. RetroRenovation did a good roundup of sources for vintage-style bathroom tile a couple of years ago that includes not just 4x4 squares.

https://retrorenovation.com/2019/01/22/12-places-to-find-4-x-4-ceramic-bathroom-tile-in-vintage-colors/

Tape around your toilet, then pull it please. I'll bet you'll recover some tile for repairs and they match perfectly.

My brother did what Joseph said and it worked. In a different bathroom he pull out a cabinet that looked like it was added later by previous owner and was able to get tile from under there.

We were fortunate to discover our original bathroom floor under a layer of peachy 60's floor tile.
A couple spots were broken and we also harvested tiles from under our fixtures. There are a couple long cracks that we've chosen to embrace ;).

The grout that you see is probably exactly what was there. They are probably set in a thick bed of concrete.

In some places we used Maipei Charcoal Sanded Caulk (tube) and found it a worthy match for touch ups.

We went through a couple rounds of deep clean and it's good. My only issue is they look positively beautiful after I've cleaned them and they are *slightly* wet. It really elevates the look.
I've never had the heart to put some kind of sealer on it to see if I can get them to look like that all the time. I have Miracle 511 Seal & Enhance on my short list.