Just a heads-up for anyone still using talc: you might want to check out the
FDA Warning & Recall related to talcum powder. Some products were found to contain asbestos, which is why the shift to cornstarch-based powders became more common, not just for babies but across the board.
That said, if you're using it strictly for tires or tubes, there are still non-cosmetic grade talc options online (as others mentioned), but it's worth being aware of why it's gotten harder to find the pure stuff in stores.
Better safe than sorry.