To pair with their 9mm pipes, most notably the Dublin Filter series, Peterson offers these branded 9mm charcoal filters. They fit easily into any accommodatingly drilled tenon. Made in the EU, they come in either 40-count or 150-count boxes.
From the Box
"For the driest, cleanest, and most flavourful smoke, we encourage using a filter but once before replacing it with a new one."
Pipe Nook Notes
I personally only use charcoal filters once, to get the most pristine, unaffected taste possible from the tobacco. However, I've been told that others have good luck with drying out charcoal filters and using them up to 4 times before throwing them out. Your mileage may vary. All of the charcoal filter brands I stock perform very similarly and very well, and I recommend them all or I wouldn't carry them. But I like to have as many brands available as I can, in case one or two of them runs into supply shortages.