glass/crystal cubes/blocks advice and sources

I just picked up an xTool F2 Ultra UV (5W) and I’m looking forward to try interior engraving. For those of you who’ve been doing this for a while, I’d love your advice:

• What type of glass/crystal have you had the best results with?
(K9, optical crystal, soda-lime glass, etc.)

• Where are you getting your crystal blanks?
I’m looking for good sources for cubes/blocks — especially for 3D engraving.

Also, just a heads-up before anyone says “check xTool.com”: their K9 crystal blocks are currently out of stock and not expected to ship until February. They are also very expensive, so I’m hoping to find reliable alternatives.

Thanks in advance — any recommendations or lessons learned would really help!