That's a really nice cassette deck. I wouldn't count on finding one used, and if you do it will probably be very expensive. I would try 2 places first. Technics sells quite a bit of pro gear. That's a good thing because pro gear tends to be repaired, and not just thrown away. Try calling them directly. Its possible that they may still have some parts left in stock. You can also try a music store that fixes gear on site. They most likely have access to part suppliers that don't do business with the public.
If the door isn't badly damaged, maybe you can repair it. Sometimes you can even get away with removing the glass/plastic piece altogether. If the actual frame itself is not bent or broken, it will probably still work.