I prefer the 3M model 9211.

[img]http://multimedia.mmm.com/mws/mediawebserver.dyn?6666660Zjcf6lVs6EVs666TSQc7rrrrQ-[/img]

It comes individually wrapped so it stays clean, packs flat, and has an exhale valve that improves comfort.

You can get them in boxes of 10 units for about $15 at various online sources.