I made my own.  I haven't had anyone tell me I had to use them, I use them because of the MH's weight.  I don't want one or two of the levelers to settle causing the MH to loose its level after I'm set up.
If you take a full sheet of 3/4" plywood, it will cut into 18 pieces 18" x 18".  Then using Tightbond III waterproof glue, you can glue two stacks of five and two stacks of four pieces together.  In each stack I used five stainless steel screws to pull them together while the glue was drying.  I used the green pressure treated plywood and then used additional treatment on the cut edges after the glue dried.
I screwed a 1" stainless eye hook in the edge of each one.  Then put a stainless hook in the top end of a push broom handle to grab them with.  I can either push them under the jsck with the handle or pull them out with it.  The threaded end of broom handle also fits my squeegie.
They're slightly overbuilt, but I don't expect to have to replace them anytime soon.
Bill