The different types of filters have always been available in kit form with O-ring, shim, gasket, and crush washers. I believe some of the filters are now only available in kit form. You use to always be able to get any of the parts separately. You still can order the O-ring, gasket, and shims separately as far as I know.
Personally I have seen too many aftermarket filters ruin engines to ever save money on filters. I think BMW is now trying to get the price of their filters to catch up with KTM? Maybe one of these days I will research which aftermarket filters work and which ones don't.