Not to take any business away from Jeff but, I buy mine direct from K&N. The main reason for this is I haven't found anyone that stocks the filter I use. From the K&N website you can see the dimensions of all the filters they have. Angled flanges, tapered filters for clearing certain pipes, different length filters, and different flange openings. I can pay them direct with a CC and shipped to my door. Just a convenience thing.
You can also run an oval K&N filter with the stock airbox, lid on or lid off, if you have the aluminum adapter that fastens inside the airbox. This would be a good ides if you trough a lot of low creeks or big mud puddles.
SP