Just build one. Get some 3/8" or 1/2" MDF and go to town. Get some guides online, and all you have to do is take some measurements for whatever speaker you have, how much air space it needs for its size, and stuff like that. They you build, put some carpet or something over it and your done.
Trust me, if you do it this way it will always sound better then any box you can buy - ANYWHERE - and that goes 10 fold for something you would buy at Wallmart.