i got a good one from caraudiodirect.com but there is other ones out there, depending on your amp...
upto 500w - .5 farad should be enough, about £50
upto 1000w - 1 Farad minimum - £100
over 1000w - 1.5 - 2 farad £150 upwards...
note that these are watts in RMS, not 1000w max or peak...
digital voltage displays on them are personal preference, cheaper without them, better looking with them,...
and ground ( as above ) with your amp, or youll end up with ground loops etc...