If you have a physical D100 die, just:
Roll it.
Read the number from 0 to 99, where 0 is equal to 100.
If you don't have a physical D100 die:
Take 2 10-sided (D10) dice.
Assign tens to one of them and unities to the other.
Roll them and read numbers, e.g., the result of 3 and 5 will be 35.
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