I make them. The process is a little involved and includes some hardware, a little machining and a very thin plastic material that won’t crease when crushed. Here’s some information about commercially available masks with exhalation valves, though.
https://www.fda.gov/medical-devices/personal-protective-equipment-infection-control/n95-respirators-and-surgical-masks-face-masks
[Excerpt...caution:]
“Note that N95 respirators with exhalation valves should not be used when sterile conditions are needed.” [Because they’re really designed for industrial use—not for medical workers.]
https://www.honeywell.com/en-us/newsroom/news/2020/03/n95-masks-explained
I’m guessing that small, light ball valves in very short tubes could be made and used as an alternative. They would make a sinister clicking noise between exhales and inhales. ;)
*sinister clicking noises*...’Luke, I am your father...’