It should be in the ceiling section of HD or Lowes. You don't really need suction cups you can just wedge it into the tank but you do not want to this if you have acrylic.
One way to use it as a tank diveder in a tank containing larger fish, while still having it removable is to silicone in four plastic rods, two on each side of the tank, just far enough apart to slide the eggcrate down between them.
It's usually about 10.00 a sheet. Some of the places have two differant types with the secound bieng about 5.00 more...(not enough differance to justify the cost IMO.)
Costs about 10-12 bucks. If you measure and cut it properly you should be able to get it pretty snug. Depending on what you are dividing it may be sufficient