RELIABILT 4-in x 10-in Floor Register Net Trap Black Register Vent Filter (2-Pack) in the Air Filter Accessories department at Lowes.com
Vent Assist 4-in W x 10-in L x 0.3-in Cut To Fit Air Filter (12-Pack) in the Air Filters department at Lowes.com
PURAFILTER 2000 Vent Filter (12-pack) PFVENT-12 - The Home Depot
GCGOODS Air Vent Filter Kit, 20" x 90" Electrostatic Media with Installation Tape (36+ Filters per Pack), Registers & Grilles Airduct Filters for HVAC, AC & Heating - Amazon.com