........Best place to obtain these ferrite snap-ons?........
I've been getting mine from newark dot com. I've purchased lots of ferrite stuff from them. Very fast shipping.
On my vertical (80m-6m) I am currently using 4 or 5 (don't remember off the top of my head. it is what I had on hand at the time) #31 2.4"OD donuts and RG8X looped 5 or 6 times through the stack (again, not exactly sure and since it is raining and lightning outside, memory it shall be). When I replace my coax (hopefully soon) I have a "string of beads" (not snap-ons) of #31 on a section of RG213 to replace the donuts.
Read Jim's paper. It is worth your time.