PU/PUR air filter foam in sheets
cut your own with scissors/scalpel/wad punch and use PU glue if you need to make tubes
UNI FILTER 9mm in either coarse 30PPI, medium 45PPI or fine 60PPI grades
500mm x 500mm $25
1000mm x 1000mm $50
EDIT: don't forget filter oil and a squeeze!
PU/PUR = polyurethane