I’m guessing you’re currently on Windows, right? If so, Virtualbox is perfectly fine, otherwise, if you’re on Linux already, you could use Boxes or Virt manager that are more performant, but don’t stress yourself if you just wanna play around Virtualbox will always be more than sufficient.
As for the ISOs you can just get them directly from the distro websites or, if using Boxes, you can choose one at VM creation and it’ll download it for you