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

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

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

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

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 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.