I've had success buying them in bulk from resellers, whether you trust that or not is up to you but it's worked out well for us so far. Another good option is buying an elastomeric respirator, it's much cheaper in the long run and usually provide better protection. Downside is that they often have exhalation valves and thus don't provide source control (though most people aren't protecting themselves anyways so tbh it's kind of hard for me to care about them at this point) and, frankly, you'll probably be harassed wearing one. There's more low-key options like the envo but they tend to not fit great in my experience and are more expensive (though probably still less expensive than buying disposable respirators).