I suggest you put your bike year/model and location in your signature. It would make answers a bit more specific. Are you talking about motorcycle air filters or filters for a tractor? If you are talking motorcycle air filters, there are essentially three manufacturers.
1. Emgo
2. K&N
3. UNI
Emgo filters have non-cleanable/non-reoilable paper filters. View them as disposable. If you ride 1000 miles per year, these are a good buy as they will last you a number of years. If you want the longest lasting, K&N pod filters come with a life time warranty but you can buy a BUNCH of sets of Emgo filters for same price as one set of K&N filters. The K&N filters can be cleaned and reoiled. Last are UNI filters. These cost MUCH less than K&N filters and can be cleaned and reoiled but are foam and won't last forever... a few years perhaps. So the choice on which to buy depends on how much your ride and your budget.
www.z1enterprises.com sells both Emgo and K&N. JCWhitney sells UNI. If I had some idea what bike you were asking about, I would tell you what size was needed.