You can trade cores for prisms at Rahool or buy old armor from Ada & masterwork it to turn shards into prisms.

Agreed

Agreed. I'm playing through campaigns on a newer character right now and that's gotten me more than a hundred, but nothing repeatable.

Agreed, but I'll tell you the best source I've found: Solo Ops give 2 prisms per run. I don't think it has to be ultimate, but that's what I run. And I can do one in 3-5 mins with objectives. I would guess Master+... and that could be done quicker. 2. If you have a good stock of enhancement cores... when you're in that limbo state, where you're at max glimmer... but nothing to currently upgrade? Buy Prisms. Buy 10-20.