I have. (It was an old credit card.) Expired credit cards and insurance cards have a good thickness to them. Perfect for a gritty, hard plastic-on-steel sound.
But they’re a little unwieldy. Not quite pick-shaped. So check this out–guitar players can use this “pick punch” on Amazon to make their own guitar picks out of just about anything. A bit pricey, but an interesting novelty item nonetheless.
Words on the Word 
I’ve definitely used credit cards, coins, those square plastic bread-bag fasteners… The main reason I’ve tried to get better at playing with my fingers is because after 30 years of flatpicking, I still can’t keep track of the things. But along with being expensive, a pick puncher isn’t going to travel too intuitively… Just bring a pick! Sounds so easy.
You’ve been playing the guitar for 30 years?
Yup. My dad started me on lessons when I was two. It was a two-string plastic guitar, but I learned I-IV-V7 in C on it–top two strings, one finger on the first fret–along with the conceptual hurdle of singing and playing at the same time. Not that I was saying intelligible words yet…
That’s pretty impressive.
Maybe, but mostly on my dad’s part–he’s a talented teacher. You could have learned that young too, I bet. We could have been talent toddler-pals!
Indeed!