Ikea, and specifically one of their APTITLIG models:
which I have found to be the most consistent. It’s 9.5"x6"x0.5", I must have cut tens of those by now and never had any “surprise” because the inside looks exactly like the outside due to the nature of the bamboo strips glued side by side.
I stay away from the thicker models (from ikea or elsewhere) that look like this:
because the once you start milling deeper than that upper layer, you uncover the inner filling which is usually very low quality bamboo with holes and spots.
My absolute favorite stock is this block (from European retailer “Habitat”) of endgrain bamboo:
which I used for example for this and this project. It’s 1" thick, and I just love the look of a freshly cut endgrain bamboo (lots of minuscule holes which give it a very specific look)
I have yet to find a local supplier of “raw” bamboo stock so until then, Ikea it is (also, I just love the look on the Ikea person’s face at the counter when I scan 10+ cutting boards and nothing else )