[mindslight]

I had them before the pandemic, but I've seen them in stock quite regularly. Zoro appears to have my 3M 7503 half-face piece and 2297 filters in stock right now. For welding you might want a proper vapor filter though (60921?). 3M has some nice pdfs that list all the different options.

For pandemic usage when I need to filter my exhale, I disabled the check valves by popping the mask apart, removing all 3 valve flaps, and sealing up the output hole. This makes air flow through the filters bidirectionally. I've also seen a 3d-printed adapter for the 6100 series (I think) that attaches a third filter to the output. Separate input and output filters is probably a better way to go.

[stirfish]

Thank you. I'd never heard of zoro before, but I'll be depositing my paychecks here from now on. It looks a lot more fun than the bank :)

[mindslight]

Haha, yes it is. They're the customer-facing brand of Grainger so they've got everything. Their search is okay and some of their product data can be lacking, but their selection is massive. As I alluded to, it's often worth it to go find the manufacturer's pdf catalog and then search by part numbers.