Yeah well it would help if you provided more info ..
Like what bike and carb?.... orrr .. what diameter carb inlet ?
Motorcycle shops can order small Uni-Filter brand foam filters in small sizes... one may fit nicely on yours .. (you have to measure the air inlet).
Otherwise... can't you try one of the moped suppliers listed here for an original air filter for yours ? .. (new or used)
Other than that you will have to be creative...
... maybe use an old 'metal window screen and bend it to the necessary shape to fit over the carb inlet ... and then buy some 'filter foam' at an autoparts store and cover the screen with that ?.... then hose clamp it to the carb ?