A Multiple Listing Service (MLS) is where all of the realtors in a particular area get together to share their listing data so they can help each other sell the properties they have for sale.
In Madison county, it's still done the old fashioned way, where you need to call each individual realtor to find out if they have any properties similar to what you are looking for.
On the other hand, there are some agents who are part of the NW Arkansas MLS (realtors in Washington and Benton County) who also have listings in Madison County (I used to have 60 acres of hunting land listed near Pettigrew a few years ago--it was sold, of course). Thus you can search in the NW Arkansas MLS for land. The Eureka Springs MLS also shares data with the NW Arkansas MLS so an agent from Eureka who has that type of land listed may also appear there. You won't get everything that's on the market, but you can find a lot that way.
I have a consumer website that has all of the listings in the NW Arkansas MLS. See the URL in the Web References below. Select Land as a property type and your price range. You might want to put Huntsville for the town, since a lot of the listings for Madison County are under Huntsville. Also a minimum number of acres that you want.
I can also check with the people I sold the Pettigrew land to a number of years ago to see if they are interestsed in selling. Contact me on my website below if you want me to do that.