Thin dry and soft. Pine, Douglas Fir, Poplar. I recommend staying away from Gum, Oak, Macrocarpa etc.

We want the fire to burn hot and fast. Harder woods burn to slowly. You may get away with using some once the sauna is up to temperature, but if you want to get up to 100 degrees, go with something light and dry.

I struggled to find a local supplier who could keep up with the amount of wood we were going through. One cubic meter per week. At this volume it's no longer an option to handle the wood twice, split the larger pieces, discard some because they have too many knots. We now use kiln dried Douglas Fir for consistently dry and small pieces. Douglas Fir doesnt grow as wide as other trees so its often nice and small already, no need to cut it by hand again. It's more expensive than pine but boy is it good.