A person owes me money. He can't pay. He is willing to put a lien on his house for the debt. Where can I find a free form to file a lien on real property in the state of Arkansas

Its not that simple. A lien just can't be filed without court action unless it's a mechanic's lien meaning that some work was done on the house for which you weren't paid.

What you probably need to do is get him to sign a promissory note giving the property as collateral. Then you can use that note to file a lien.