I currently use AO Safety Woodworker's Respirator #95190 (Amazon.com) with replacement filters. I remove the filters and pat them out until they just can't be used any longer.
Its 4.5 stars out of 5 in 36 reviews on Amazon, and its $15, light weight and doesn't fog up my glasses. It does not fit well under a face shield is the only drawback. I wear it under a face shield for a couple minutes whenever I'm rounding a peice of dymondwood though.
I turn a LOT of dymondwood and other stuff that doesn't go well with my breathing, so I always try to have the mask on.
When I ever get around to posting pictures of my shop, you'll see I have no ventilation. If I didn't have this mask, I wouldn't last long out there. I switch over to a 3m pesticide mask when I spray finish to end a day in the shop.
If you go to the reviews, you'll see I gave it a 4 out of 5, wishing you could breathe through the filter easier.......but this is a whole lot cheaper than one of those huge masks like Al is talking about that would hurt my neck.
Parker