Here's the thing about thumbpicks: They have to be relatively stiff/thick so that they will stay on the thumb. Also, they aren't so great at up-strokes, since that tends to knock them off the thumb, since the strap doesn't wrap all the way around the thumb. Of course, using your index finger to back-up the thumbpick will help keep in on the finger.

There is this device that allows whatever flat pick you want to become a thumbpick (note: you'll see other wrap-around thumbpick devices at the top of this linked page. note that none are thin picks)

https://www.amazon.com/Strum-N-Comfort-SNC-T4-0-DP-Sharktooth-Traditional/dp/B004LYGAA0

I've never tried the "Strum-N-Comfort device, and I have some doubts about it (fear that the velco wrap will contact the strings), but in theory it would allow the use of thin picks.

Others here have mentioned that Jim Dunlop makes some paper thin, nylon picks. Nylon is great because it's next to impossible to break or tear, and can be made very thin, or thick.

There's a half-dozen other materials that mass manufactured picks use, with cellulose being the cheapest and perhaps most common. Though cellulose picks are easy to get, but break easily.