I wouldn't be without one - they keep a lot of trash off the top of the slide. But if you don't use the rig much or stay in open areas where leaves and such don't clutter the top of the slide, maybe you don't need one. Some slides bring in water that may have accumulated on top (even dew) when you retract them, and if you have one of those a topper is really nice.
PPL Motorhomes seems to have decent prices on this sort of thing, but RVUpgrades.com is my favorite for accessory type purchases. The prices are fair (though not always the very lowest available) and the service is excellent.