How do you play D&D when we don’t have dice to play with?

Now I came all the way from Indonesia, and I can tell you that D&D is not a thing. Hence, it will be tough for me (and for my wallet) to find shops that sell d4, d12, d20 and so on. Let these dice alone. You rarely see people selling “just” the common d6 alone. I do not have a lot of money so buying a game and taking the d6 from it, for example, is not feasible.

So, how do you play D&D, when you have no dice to play with? I know, for d6, I can use 6 sided pencil and roll it, but I never saw a 20 sided pencil.

I’m particularly looking for a method that is hard to be manipulated (cheat-proof) and is reasonably fast.

Assume you have no access to technology. That includes everything that uses electricity. Iphone, laptop, so on.