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.
