Greylock said:
Write the numbers 1 through 20 on little even sized chits. Put them in a Yahtzee cup. Voila', you're good to go.
From what I've read, in the very early days of D&D (i.e., before polyhedrals were widely available), this was the recommended way to generate numbers beyond d6s.
In a very early issue of either Dragon or The Strategic Review (republished in Best of the Dragon, Vol. 1), there was a humor article on alternate ways to generate random numbers, beyond the chits in a bowl. Those included numbering Mexican Jumping Beans, pulling random numbers of hairs out of each other's chests (only recommended for hirsute gamers), etc.
Also...I used to play a lot of Strat-o-Matic Baseball. In that game (which mostly uses d6s), there is also a need for generating random numbers between 1-20. These days, they supply a d20 with the game. Back in the 70s, when I first bought the game, it came with a "split deck" -- a deck of small cards (about 1" by 1.5"), with a number from 1 through 20 printed on one side, that you'd punch out. IIRC, there were 40 cards in the deck, 2 sets of each number. When you needed a "split" number, you'd draw the top card from the deck, and use that result.