As to tape, did you really mean cassette tape, or did you mean CD? Most hardcopy audiobooks are on CD these days unless they are very old, and you'll only find those at your library.
The gadget mentioned is an FM adaptor, and I don't recommend them unless you are dealing with an actual cassette player-shaped one; the ones that plug into the cigarette lighter tend to fade in and out if you can get the signal at all.
You can play other devices via your car radio if you have an AUX jack on it; the cord you would need is a male/male aux cord; it looks like a headphone jack at both ends. You can get them anywhere that has headphones, and it will cost ~$3
My recommendation is the Kingdom Keepers books, they are set at WDW, so they make for great anticipation builders. They are YA, but don't contain anything that a 6 yo shouldn't hear, so they work for all ages. If you want something younger, I'd go with Peter and the Starcatchers, again, Disney-themed.
If you get CD's and are going when it is very hot in Florida, I recommend taking a small insulated lunch box to store the CD's in. CD's (and audio tapes, should you find those) will warp in very hot situations, and you don't want to ruin borrowed items.
