Aside from the K&N, the only other place that I've been able to get them are either 1) dealer, or BMW-specialty places like capitalcycle.com
I've found numerous replacement air filters from eastern asia for the oilheads, but choices are not wide for airheads, it seems.
I'm still running a "stock" air filter, but it wasn't cheap and I may "upgrade" to the reusable K&N this year or next.
... Just checked the purolator website - it is pretty useless. They do have a motorcycle filters document, but it appears to only have oil filters, and for that matter, I don't think that the application information is correct in it.