Supplemental Health Insurance in the United States-- Do you have it?

I have health insurance from my employer but medical devices aren't 100% paid for; I am responsible for 20%. It's not terrible, but last year (through the marketplace) I was covered at 100%-- although the plan was very expensive. Does anyone have any experience buying supplemental health insurance? Who did you purchase from? Do I have to wait until open enrollment though the ACA? I'm asking now because open enrollment in my company's benefits is happening. Normally, I would go to HR but honestly, our HR dept is a hop away from being useless (only because they are overworked).