Filters for large commercial coffee makers are sold online at KaTom.com and at Amazon.com, as of March 2015. Filters as large as 25 inches in diameter are available for large urn-style commercial brewers.
KaTom Restaurant Supply sells large Bunn-O-Matic coffee filters in 250-count packs for 3, 6 and 10-gallon urn-style coffeemakers. Grindmaster-Cecilware filters are also available in 500-count boxes of 21, 23 and 25-inch diameter filters. Most orders ship within 24 hours.
Amazon sells 250-count cases of Bunn 18-inch diameter filters for 3-gallon urns and 23-inch diameter filters for 10-gallon urns. Cases of 252 Bunn U3 18-inch filters for U3 commercial 3-gallon urns are also available. Amazon offers free shipping on orders more than $35.