I played with 3D engrave for a spell. So much so (about 2 dozen or more) that I had to clean the machine twice before I moved on to other things.
A good resource is available at Home Depot in their wood floor department. The little 4x4 samples are free and I had been getting a few samples each time I went.
Just make sure you are getting the wooden floor samples, not one of the vinyls.
Not a great candidate for finished projects, but a free resource to allow playing with your files until they look good enough to put the good wood under the laser.
(NOTE: Old wooden fan blades are also nice to work with - MDF centers on the wood ones can make them harder to cut than real wood, but they are also another freebie if you have been like me and replaced fans over the years and hoarded the blades)