You might all know this already, and if you do I apologize in advance. But if you don't here is a way to make your own oiled-foam air filters.
I learned some years ago, a pretty useful trick. Buy some UNI air filter bulk foam. Local motorcycle shops will order it for you or go online. It's cheap and comes in sheets of green foam. Get out your sharp scissors and make whatever parts you need.
The secret is clear silicone RTV adhesive. It will permanently glue the air filter foam to itself. To make this one for my Urban Express, I cut a piece to fit around a 1-inch diameter pipe, glued it and held in place with painter's tape until it dried in a couple hours. Then I made a round piece for the end and glued that one.
This kind of foam needs to be oiled to be an effective air filter. So I shot it with a few blasts of aerosol Spectro Foam Filter Oil. Pretty much any kind of oil will do in a pinch. Clamped it on with a zip tie because I didn't have any clamps small enough. It's not pretty but it will work great for a long time if I need it to. Better yet it's washable and re-oilable and it cost almost nothing to make.
Pretty cool, huh? (edited)