Where can one get replacement tips for the S Pen?

Most tips will still work, but the phone (if unopened) should include 3 replacements (might be 2, can't recall). As for replacement, Amazon would be your best bet (Samsung sometimes has them listed, but tend to disappear or have huge lead times).
 
Got mine by calling Samsung Technical Support. Took about two weeks for them to get here but the tips for my Tab S7+ got delivered today.