Looking for a blank polished stone

So over the weekend our beloved dog died, and we are having her cremated. I plan to find a quiet spot on our property to bury the ashes and I would like to engrave a flat headstone for her. I can't seem to find a reasonably priced considerate option. I see like $20 ones on Amazon but they don't come blank or large headstones for hundreds of dollars. I am just looking for like a 6X12 inch flat polished stone, maybe granite or marble that I could engrave. Thank you if you have any suggestion