The receiver should already be drilled and tapped. If not you can get a bolt on cantelever mount that attaches to the vent rib or a saddle style mount ytat fits around the receiver.
I ordered a Picatinny rail through Havlin Sales. They include the screws that fit the 935. It was $22.50 and $8 for USPS priority. Now, I need to decide which red dot to get. I'm considering the Burris fast fire.
I ordered a Burris Fast Fire III. It ought to be here Monday. The Havlin Picatinny rail works for me. From what I'm told, you can use any Mossberg 500 rail, but you just need the proper screws to fit the receiver.