What size and type filter can I use in the filter pocket version of the mask?
The filter pocket versions accommodate 80mm x 120mm filters. People commonly use PM2.5 filters, but even paper towels or other layering has been shown to add more filtration capabilities.
Where can I buy filters? Does NxTSTOP sell them?
NxTSTOP does not include filters with the masks, nor sell them on the site. However, they are widely available on Amazon.