What are the main ways for witch to get power charges?

Specifically in scenarios where there's no adds and thus no corpses.

Profane ritual is the main skill but requires corpses to use.

To use that in single target boss fight I would need to spend 60 on the spirit gem that lets you sacrifice your own minion's corpses and as well as some spirit to get at least 1 minion.
Which i assume is no problem for minion builds but I am mainly going for non minion build and want to at least have access to convalescence.

Are there any other ways to get power charges or is that it?

There isn't really any other way that is aimed for witch. I guess the other best option for your situation is lingering illusion if you get the dex. Then just dodge boss attacks carefully to get charges. It has lot of chance for additional charges too so you can pretty much get 2 charges everytime your illusion gets destroyed. Use persistence on it to make the timing easier.

Voll's Protector (Body Armour) - 25% chance to gain a Power charge on Critical Hit

Redflare Conduit (Body Armour) - 20% chance to gain a Power Charge on Hit

Sniper's Mark + Resonance Keystone - Gain a Frenzy Charge off the mark, then instead of gaining a Frenzy Charge Resonance make's it a Power Charge