Cassettes are considered obsolete, so there are a lot of really good cassette recorders 20-30 years old, that you can buy cheap today because almost nobody uses cassette anymore.
Back in the 70s, we used to have cassette 'decks' that you would plug into the back of a stereo. They didn't have an amp or speakers, so they wouldn't work stand-alone. But they were high-quality, fairly expensive for the time. You can buy these on EBay now for <$100--a really excellent deal if they work at all (or Craigslist, or even garage sales). You can plug them directly into the Line In input of a computer and use a free program like Audacity to record them into WAV files, and then convert these to MP3. Brands like Teac, Sony, Nakamichi, Akai, etc.
Do a search for 'cassette deck' on EBay and you'll see.