I found one on the K&N site that would fit my 48cc bike. There is a way that you can search for filters based on dimensions. I don't remember the part number now and I ended up not ordering it. I opted to get the CNS carb because it already has the type of filter that I wanted plus a few other really nice features.
I think the round straight ones at the top of the page are the ones that you need. Just measure the round part of the carb where the filter will attach and search for one with a flange inside dia. that is the same or very close.