how would I source the needed strontium-90/radium/other rapid-decay isotopes to build a RTG?

step one: enroll in a university and become a scientist
step two: build a time machine
step three: travel to the future and steal the radioisotope thermoelectric genorator from your future self
step four: travel back to the past and reverse engineer the radioisotope thermoelectric genorator to make more
step five: wait for your past self to show up and steal the radioisotope thermoelectric genorator from you