For synthetics I have two go-to vendors.

https://store.turtleshoard.com Turtle's Hoard stocks a lot of weird, interesting, science-y facet worthy materials. The Christmas Garnet is worth a gander, I've got a piece of that I'm holding on to for a rainy day.

https://www.etsy.com/shop/TomsBoxofRocks Tom sells more "traditional" synthetic material, like corundum, CZ, and nanosital. He has a lot and you can find some cool color change and bicolor versions. I find his corundum is competitively priced with Joe Henley so check them both when you're shopping.

I had this vendor recommended to me by my mentor, and recently bought from him: https://www.westerngem.com His website doesn't display well on mobile, the pictures rearrange in strange ways, it's better with a desktop browser. He sent me a complimentary citrine when I placed my last (rather large) order. Everything was exactly as described and the topaz was even more blue than the picture suggested!