The blank cards they sell are great. We borrow audio books from the library and I rip them to a card, you can reuse them as well so don’t need to buy too many. I also put radio streams on them, like classical stations for when my sons going to bed.

You can use third party cards which are sold for a fraction of a price too. There are a bit of hassle to setup (you need to link an original card and then clone it to a cheap card), but when done they work flawlessly