Hi,
You can get black cherry seeds from the store bought black cherries in the produce department - if they are available in your area. So long as it is fresh produce - not frozen at any point - you can use the seed - including cherries, and any other fresh produce - tomatoes, peppers - green, red, yellow, orange, jalapeños, apples, oranges, lemons etc etc etc.
I don't know if this helps. Walnuts have been baked before they are sold so those you need to source.

Cheers,
McPeg

PS I work in a grocery store and I get lots of seeds from produce that is used to make salads...I get the gals to hold the seeds out of the garbage for me.