I’ve ordered from three of the companies mentioned. As George says, CurlyMapleWood has beautiful stuff, but no veneer. But if you’re looking for curly, or torriefied (roasted) maple or sycamore, they’re hard to beat. You can pick exactly which board you’re going to receive, and there’s generally some bonus thrown in the box, as well.
CertainlyWood has beautiful veneer, and you can pick which specific pieces you’re going to get. I have a stack of ash burl veneer from them that will one day become a five foot diameter tabletop, but I need to up my veneer game first. As mentioned, you can pick your thickness, which is helpful.
Veneer Supplies… that’s where I’ve been getting the veneers I’ve been practicing with. I got two sampler packs / mystery boxes, and it’s a great price for good veneer, but it’s almost all been thin stuff. 1/42” IIRC. But for a beginner practicing? Go for it. Buy a couple mystery packs and knock yourself out.