Either use car gear, or even better is to use PA gear, since those often have crossovers in them since that means you dont have to try to power the car gear.
Since the subs share airspace you will need to combine the signal to mono and then put that into both of them to stop them from going nuts when fed out of phase bass.
Look on ebay in the musical instruments catagory for PA gear - similar power levels in "home audio" gear usually come with the Audiophool pricetag to match.
Otherwise you could use a home theater reciever and set the main speakers to small, this will highpass them and then you will need a seperate power amp for the sub - jaycar have part AA0508 - I am sure they used to have some smaller ones as well, possibly discontinued - there is a much nicer selection of things like that in the states then aus/nz...