You can get the laminated cardboard signs almost anywhere. Try Home Depot, Lowes, Office Depot, Staples, Walmart, etc.

My grandfather had a large piece of plywood that he painted white, and then stenciled on the information needed. This was attached to two large posts in his front yard. His property was on a prominent hill alongside a busy highway.

You can always buy a generic FSBO sign at Lowes.

I had a custom sign made with my phone number and a couple lines like "fully remodeled" "3.5 acres" and the website address to see photos. It looked a lot more professional. I don't recall where I bought it, somewhere online, but if you google "fsbo sign" I"m sure you'll find lots of options. I bought a metal sign holder as well.

Whatever you do make sure the contact information is LARGE and LEGIBLE. Imagine trying to read it from the street.