UNDER CONSTRUCTION
The word ‘free’ in the title is in quotes because many of these sites (and designers) ask for donations if you print and keep the cards on your machine. If you do keep any of these cards, please donate to the associated artist/designer and/or websites. A lot of work goes into some of these cards and running the websites, and their generosity should be reciprocated.
Websites
https://www.pinballrebel.com/pinball/cards: This site has a large collection of Rule cards. Many are the original cards in digital form, but there are some custom ones as well. I believe the website owner asks for donations if you decide to keep the cards (on honor system).
Pinside and IPDB: Both of these sites have some original cards in digital form in their respective game image galleries. On Pinside you can also try searching for something like ‘custom score cards’ along with the game you are looking for; sometimes people share quick versions of score cards with art in the background (usually just with the original text from the original cards though).
Specific Games:
Jurassic Park (DE, 1993): https://pinside.com/pinball/forum/topic/free-custom-jurassic-park-instruction-cards
TMNT (Stern): System-J Legacy TMNT