who do i contact to make custom plastic cassette cases and shells?

I agree with lewm…this forum ceased being helpful long ago. If you want snarky opinions, you’re in the right place. 
Check with the guys over at tapeheads.net. However, what you seek may be very hard to find. 

Why would you even think of doing this? Anyway you’ve come to the wrong place with your endless questions about a dead medium. There are actual forums for tape junkies. Find one.

I think you need to contact the 1970's :)

@leemurray2007 Have you contacted ATR Magnetics?
https://www.atrtape.comTo my understanding they are the only US manufacturer still making cassettes.

Several labels offer their artists on cassette. Try contacting them 
www.newwestrecords.com 
www.barsukrecords.com
to name a couple

party pooper..

I vote thumbs down.

Unless you can find the original injection molds for those cassette cases your in for a rude awakening on having new molds built. Plastic injection mold are not cheap. The original molds were probably at least 24 cavity and designed for a fast cycle time.

I would look for an existing manufacture, probably in China and talk to them.
You would need a huge market for the finished product to even think about starting from scratch.
Where will you source the tape from? High quality recording tape isn't commonly available these days.
BillWojo

Fuji for one.. LOL are there Copy rights? I'd doubt it..

Don't stop there. Reel to Reel to.. Pre recorded with a GREAT new medium all for a reasonable price.. You got my vote..

Regards