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

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

I vote thumbs down.

party pooper..

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.