Years ago we called plungers "Plumbers Helpers" but in todays oversensitive world that is no longer politically correct. (It seems that the plumbing apprentices take offense when we call a plunger a "plumbers helper"...LOL)
Sink plungers are designed to work on a flat bottom or slightly curved bottom sink, but just as you stated, they cannot seal properly to effectively plunge a watercloset(toilet).
To plunge a watercloset you need a "Closet Plunger" which is basically the same as a sink plunger but it has an extension on the underside that has a concave shape to fit into the watercloset trapway. (see attached illustration)
You can find a closet plunger in any local hardware, home supply center or your local Walmart, K-mart etc in the same area as the common sink plunger. One advantage of a closet plunger is that the extension piece can be folded up into the upper section, then it will perform the same as a common sink plunger.
Even though a closet plunger will perform as both a closet plunger and a sink plunger if you happen to have a double sink in your kitchen I would strongly suggest you have both types handy because when plunging a double sink you must cover the drain opening in the sink opposite to the one your plunging. If you have two plungers it is very simple to cover the opposite drain opening with the second plunger.