You need too talk to an attorney so that the option is recorded that will prevent the owner or if the owner dies their family from Refinancing the property or cashing out you don't want that to happen. A Realtor may assist you but if you have found the property and have the price worked out just go to the attorney if you would like a recommendation send me an e-mail.
If you elect not to talk go the attorney route you may buy forms online and file at court house yourself
Cost of attorney is $350+/- plus recording
P.S. YOU HAVE TO RECORD THAT OPTION FOR YOUR PROTECTION