How to Pick RAM to Work with Existing RAM?

I would like to expand my RAM to 64GB as I have two free slots in my motherboard. Unfortunately the kit that I already bought seems to be discontinued and I can't find it for sale anywhere. How can I choose another set of RAM that will work with my existing RAM?

My existing RAM is a Crucial Ballistix Sport LT 3200 MHz 16 CLI 2x16GB kit. My motherboard is a Gigabyte Z390 Aorus Pro.

Most answers I can find on Google say not to mix kits, but I would really prefer to be able to keep my existing RAM and just add on to it rather than having to buy a full 64GB kit.

Thanks for any help!