In my opinion, the best cassette deck is made by sony.
Pyle makes a cassette deck that is around 80 dollars.
There is the Ion Audio Tape2PC USB Cassette Deck.
You can always connect your cassette deck to a computer via a mini-to-mini cable.
Any of the Sony Walkmen will be great for recording classes.
You can buy dubbing cassette decks and they'll work well for converting CDs to cassettes.
There were 2 different Cassette players available from the factory, but since the Reliant was discontinued almost 20 years ago, going to a store like Best Buy and finding a Cassette Deck is your best option. But finding a good working Cassette Deck online (like eBay) for any Chrysler/Plymouth/Dodge (except for the Colt) from 1984 to 1998 is also an option. Those are all interchangeable with the one in your '88 Reliant.
Let the dealership deal with the battery after you buy a better car with a CD player instead of a tape deck,
Hot Topic, FYE, or Best Buy are the best choices.
You can purchase a used CD radio and cassette player from Adorama, Best Buy, Amazon, or eBay. You can buy it for a cheap price from these four places.
Cassette adapters are very cheap and can be found at Best Buy, Target, or any store with a electronics department. Look to spend between $6 to $12.
best buy
Should be 4 small holes on either side of the deck. You can buy a tool, or use a coat hanger. Insert the tool into the holes, this pushes the clips in, then slide the deck out. Lots of pictures of this if you google it.