cheapest would be build your own. really need the "claimed" peak power of the amp - not the sub. to work out guage for the positive.
decent car parts will sell battery wire that you can use for the earth.
again they wil also sell 5amp wire that you can use for the remote lead.
and ebay for phono leads
ebay for the fuse holder and fuses
or if you do want to buy it as a pack the likes of halfords usually have them