I haven't seen one. Most likely, you will have to make your own.
The least expensive way is to use cardstock and your computer printer. Check your printer for how thick cardstock it can handle. You should be able to align the printing so that you can print on both sides. Make sure you don't have narrow borders so any off center printing or cutting isn't apparent. If you do that, I'd recommend using a paper trimmer and corner rounder:
https://www.amazon.com/Fiskars-152490-1004-SureCut-Deluxe-Trimmer/dp/B0038F1A0S
https://www.amazon.com/Sunstar-Kadomaru-Corner-Cutter-S4765036/dp/B0076FJ7SS
You can check museum or zoo websites and see if they sell postcards (a lot are closed now, so you can't access their shops) but probably the best bet is to go somewhere in person. Also, there are a lot of websites (e.g. Zazzle) where you can buy individual postcards and you can get a bunch of them to construct your own deck. When buying postcards, make sure the backs are all the same (you can customize on websites like Zazzle). Again, you would need a paper trimmer. Expect to pay around $1 a card -- a 30 card deck would be $30. The benefit to this is that it looks less contrived and you can use where you got the postcards as part of your presentation.
There are some "memory" card games (where you have to match two cards) to there, but you would need around 8 decks of the to make a 30 card deck. That is a bit expensive. You would need the trimmer and rounder.
There are companies out there that make "custom game cards" where you can design the front and back of the cards. This is probably your least expensive / best quality option. You still will need trimmer and rounder.
