Exiting, the clothes and masks are treated as hazardous waste. On removing the mask and clothing, the wearer would then take eight showers.
We've had yearly antrax scares for as long as I can remember. I don't think a gas mask would be necessary -- however, I don't know a source for the masks the hospitals use (but I do have a handfull stashed).
If you search for "medical supplies", you'll turn up a large number of internet suppliers of gloves, surgical masks, etc.