I bought it on Bezos' Capitalist Dystopia-Mart here. They're sold as insulating material for microwave ovens.

Unlike the first set I bought, these are large enough to span the ring of a tealight burner.

u/SamsaSpoon has great recommendations. I'd also add LiaIncenses and Heyoka Frankincense on Etsy.

A tealight is pretty darn good. I got some sheets of mica from Amazon, and using them in my tealight burner is worlds better than using a metal/foil cup! I can see exactly where the resin is in relation to the flame and adjust accordingly.

Even when using charcoal, I'll go for a layer of salt on top and then a mica sheet, so that the resins and woods don't combust, but just get heated. No scorching, great scent!

These are the mica sheets I use. I also ordered some others previously, but they're too small to use over the tealight burner or full-size charcoal tablets, though they do work on medium and small charcoal tabs.