You can buy a number of replacement trimmer heads from Home Depot, Lowe's, or your Local Hardware Store.
Look for one that DOESN'T spool out more line as needed (you may be able to get this to work, but I don't know.). And attaches to the trimmer with a bolt. It should also have a very round bottom, unlike a whacker this trimmer head needs to be able to support weight and glide across the ground.
I purchased and used this one
Remove the blade from the mower, and see if the bolt head will fit through the trimmer head. If the head is too big (mine was) you need to grind off a little material from each edge of the bolt head so that it will fit.
With that done, simply screw the trimmer head on.
Balance is extremely important
, so if you install longer trimmer lines, make sure they are the exact same length.