My wife and I want to purchase one plot for two cremation urns and one headstone how much would this cost?

Can you buy headstone from a third party and have it pre-engraved with birth dates and names and have the date of death added once that occurs? How much does this all cost? Will cemetery allow you to bring your own headstone if you buy that somewhere else?
My mother-in-law just passed and had an expensive hearing aid (over $3000) Can we take that somewhere and sell it to someone that needs this?