You will probably have to find them on your own like I do. I turn the cores of the longleaf white pine cones, some folks cast smaller cones in PR and turn that. Both are equally spectacular finished.
Theres a few threads in the forums where folks describe how they do them, search and try out a few techniques for yourself, PM me if you need any specific info.
I've only done one, but it was a success. I look closed pine cones. Sometimes they open after a little drying, but it is what it is. I didn't cast mine, but I used ALOT of thin CA to "stabilize" it as I turned it. Good luck and most of all, have fun!
I can fix you up with some pinecones if you're interested...I have a virtually unlimited supply. They don't have a very big core and will be difficult to drill bigger than 7mm without blowing up. Some are even too small for 7mm. I've been experimenting with casting them with some success. Finished this one today. Drop me a PM if you want me to send you some. By the way, my wife is from Round Rock and I grew up just down the road in Hutto.