Dice, plus a computer with some linux installed and connectivity disabled. You need the computer because sha256 hash needs to be calculated to give you the 24th word. (Without that, you'd need to test up to 256 different 24th words to find which final word renders a valid seed. But it's reaaly bad to choose your own seed anyways.)The tool mentioned elsewhere, https://github.com/iancoleman/bip39, is good for this. You can save the html file in a pen drive and then, later, load it on the computer without any internet. You can use it to input the dice rolls and it will produce a corresponding seed phrase. Please look for instructions on that for more precise steps. You'll need 99 rolls to get 24 word randomness.