I have a few of these for sale $30 shipped. They are brand new in the box.
SHOP VAC HIGH EFFICIENCY HEPA CARTRIDGE FILTER FOR ULTRA FINE DUST CAPTURES 99.97% OF PARTICLES AS SMALL AS .3 MICRONS FOR USE WHEN PICKING FINE PARTICLES SUCH AS DRYWALL DUST, CEMENT DUST, COLD FIRE PLACE ASH, AND...
I went with this one last year. It was easy to install and helped a lot. I will try to get pics, if you want.
http://www.lowes.com/lowes/lkn?action=productDetail&productId=299870-278-GD01&lpage=noneI