not really sure where you can get the carpet for cheap, but to put them onto the box, i believe you can use the spray adheisive to put the carpet on. Check your local auto part store for it, they should have it.
Check at your local stereo shop first. I went in to the shop down the street from me and i told them i needed enough carpet to do the false floor on my trunk, my amp rack, and my sub and midbass boxes. He said it would be about $20 or so, and when it was all said and done I had a piece 8 feet wide by about 15 feet long for $10. Way more than enough for the job.