First up: flexatone. You can find a little history, a listing of compositions using flexatone, and some fantastic sound samples here.
While it is not used very frequently, the flexatone is one of the instruments for which there is really no suitable replacement.
Similarly, the vibraslap, while used much more frequently than flexatone, is not called for very much. Find some more information here.
As always, take some time to look into both of these instruments and see if there's a place for them in your bag of tools. If they can help you to be more expressive, creative and musical, you should get ahold of them soon!
*It can be frustrating as a percussionist or band director to try to acquire all of the little percussion "toys" that are called for, especially when you only use it once or twice. But when you've gotta have the sound, you've gotta have the sound.