I agree with BlastBass. I'd look into some combination of octave pedals and envelope filters if you're looking for tuned, trap style 808s. Correctly setting up the filter envelope is important to getting good 808 sounds. Fast attack, slow decay and release... I think... If there's an ADSR filter pedal out there that's what you'd want. The Moog seems like a good bet. EDIT: I recall that my EHX Bass Microsynth could get pretty close. It wont be as crisp as an 808 sample though.