There are many Octave pedals available in the market.
Do you know what type of sound your are looking for?
I have various of them just because of that reason... EBS Octabass has a really nice clean octave that tracks really well up on the higher notes, Boss OC-2 has a really nice presence and organic feel to it, Aguilar Octamizer is really subby and you can control the tone of the clean and octave too, and the list goes on.
As Fionn mentions, you have polyphonic octavers like the EHX POG and Micro POG too.
Just put a nice drive/fuzz after the octave pedal and you will sound killer!
Just depends what sound you are after.
Byron.