So, this happened while installing a brand new speedo cable. Does anyone sell just the plastic end or do I need to purchase another brand new cable?

I never seen the end sold separately. You need a new cable. I would try and find a mopar one. I had bad luck with aftermarket speedo cables.

Epoxy it back together, not much stress on that cable

Put a small hose clamp around it.

Or take it back and get it exchanged for better brand

super glue will hold that.

This was supposed to be a good one that Rick Ehrenberg sold on his e-bay store.

I hooked it up to the dash cluster with the dash assembly pulled away from the windshield. I think it bound up as I rotated the dash into place. After the fact I realized that it was easily accessible from under the dash and I didn't need to plug it into the cluster when I did.

Contact Rick, Worth a shot.

I can see maybe an old aged one breaking. But a new clip material should have some "give" to it.
I say flaw, or just plain cheap material.
Have seen many clips break off, but never that.

I would drill and pin both sides carefully and wrap fiberglass cloth once around it and put resin over the glass. Tape off the areas you don't want the resin to come in contact with.

I super glued it last night. After the main piece was glued to the end of the cable, I built up the area around the crack with glue and sprinkled backing soda on it to turn it into a plastic like substance. I'll see how strong it feels tonight.

I would suspect the rest of the plastic is just as brittle as the part that broke.

I'm pretty sure that ATC is the only manufacturer making cables and they call come from the same source.

use some "marine grade" heat shrink over that repair.
it has "glue" in it, and will help keep the super glue repair intact.
i'm almost positive you can get it in white, and the right size for use on that cable end.