You don't need a HEPA filter. Hong Kong has discovered that N95 masks work perfectly well. Just Google 'N95' for lots of suppliers. About US$1 each in quantity. MAKE SURE THEY FIT PROPERLY!
For a fascinating look into the trenches of the SARS wars,
read these e-mail dispatches from Tom Buckley, an ICU physician at Prince of Wales Hospital in Hong Kong. Buckley has been posting to the Critical Care Medicine list (ccm-l), and his e-mails have been collected here. Oldest at the bottom.