How can I get a replacement filter element?

That looks like a Kobalt to me...if so, get rid of it now, and get a good one (Milton) from a supply house. My new out-of-the-box Kobalt failed at the drain valve at first touch...design problem in the guts. I assume it's a water/air separator type for compressors. Be sure to install a "condensate loop" of 25-50' of hose to permit condensate ahead of the filter. Otherwise, you're just filtering vapor...not good.

Replacement elements are not available for this filter. The entire filter should be replaced after 3-5 years of use.