Just about any kart shop sells them, Dover Power is where I got the last bunch of them when they were on sale. 5 new filters and foamies will last me a whole season, just wash them each week between races and hang up to air dry. If you properly oil the filter and the foam outer filter, it will keep dirt out of the engine, either way at least one of the 2 needs to be oiled for it to do its job as a filter. Also pay attention to what type of oil you use on the filter itself, some of the spray oil is made for the nylon outerwear covers only and not made for the paper element in the air filters. Some guys I know use ATF on their foam outer filter as an oil and it works pretty well. Atf is automatic transmission fluid just in case you didn't catch that part.