The easiest way is to download an ISO and etch it. I think you can just order one directly from them too:
https://ubuntu.com/raspberry-pi and https://ubuntu.com/tutorials/how-to-install-ubuntu-on-your-raspberry-pi#1-overview I didn't see an option to order a preinstalled SD.
Though if you're just getting it to play with I'd highly recommend installing Raspberry Pi OS which is a little more lean on resources I think. https://www.raspberrypi.com/software/ Or maybe https://ubuntu-mate.org/raspberry-pi/ (unless you plan to run it headless, then I don't think it'll matter much.)
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