I’ve always just treated it as a natural 3D extension of the 2D grid rules
I believe that’s how it’s handled in D&D too, or at least how my table has always done it. I meant more as a practical matter, you’re very unlikely to have a vertical wall grid and some kind of stand of the correct height for your minis, so you can’t just count squares like you would for horizontal movement. That’s when the Pythagorean Theorem comes up in my experience.
There are lots of places that will print for you (eg https://www.shapeways.com/), but it’s cheaper and easier than ever to print your own right now! You can get a cheapo chinese printer for a hundred bucks, or an excellent Bambu for $250 (https://us.store.bambulab.com/products/a1-mini) Then you feed your model into something called a Slicer which will let you adjust all those parameters to your heart’s content (Flashprint is beginner friendly, Cura or Orcaslicer are more advanced options, all free) You’ll also need a roll of filament ($13-$20 for one roll, enough to print hundreds of minis) and then you just hit start and see how it goes! If your print fails, nbd, it’s like a fraction of a cent of plastic, just tweak and try again!
The material you want to use is called PLA, it’s cheap and easy to work with. Note that all the above prices are in USD, it’s likely more expensive to get shipped to Australia, but a lot of it comes straight from China so you never know. Feel free to message me with any questions or troubleshooting help