Some of the Treestand Manfactures, Summit, Hunter Safety Systems, offer a 3 pack of the rropes with purssik knots. They'll run around $100. for the 3 pack. Check 'em out.
The trouble with going to a "Box Store", is that they don't usually carry a rated climbing type rope.
Fire Dept. Safety equipt. sales, Rock Climbing stores, Mine Safety Suppliers, etc. will have thwe rope that is needed. It will cost more than the "box stores", but you'll get the real thing.
Be safe out there.
JAG/Johnny