Where can I get replacement cards for Ticket to Ride?

Assuming you're dealing with the original game, you can choose to purchase the 1910 expansion pack for the game. It includes a larger (standard playing card size) of all the game cards (rail cars & tickets) in addition to adding more tickets to the game... although you can remove those if you want to stick to the original game rules (as they're noted on the ticket by symbol).

With the expansion, I prefer to run the "Mega Game" variation for simplicity purposes (all tickets are in play, everybody draws 5 tickets & must keep 3)... which is worth the $20 investment & can be found where the game is sold.

I'm sorry to note that there's currently no PDF file of the cards to print out.