I used to leave some usb device with multiple bootable isos lying round my table, but I found out that every time I needed something, none of them would serve me, and I had to download something else, so I don’t do that anymore and just download and write isos as I need them. Oh, but I still keep an old 4gb usb stick with some random distro on it, just in case my pc becomes unbootable and I have to do some maintenance/data rescue.