A slap delay or slap back delay is just a short delay time. You don't have to buy a specifically named pedal to get this type of sound. Any cheap delay can give you this sound once you work out how to set it up.
There are two basic types of delay pedals available; Analogue and Digital. Traditionally the Analogue delay pedal will have a shorter range of delay lengths, up to 800 milliseconds and will sound more organic/warm. A basic digital delay will generally go up to 2 seconds these days, why anyone would want a 2 second delay is beyond me but each to their own and they also tend to sound quite clean and a little sterile.
If you want a great sounding analogue delay then go and try one of these: Electro Harmonix Memory Boy
For a cheap and good sounding analogue delay try one of these: Behringer Vintage Delay (these are unbelievable value for money)
One thing to note is that a lot of digital delay pedals will eat a 9 volt battery very quickly.
