Looks like they are available here for most 328i years:
http://www.amazon.com/BMW-Genuine-Cover-Front-325xi/dp/B007RHWZNO
Be sure to plug in the year at the top of the page to see if this is right for you.
It won't be painted though. That is kind of the tricky part.
My best guess is that it’s a camshaft seal/plug. XtremeAmazing Billet Triple O ring Cam Camshaft Seal Cover Cap for B16 B18 B20 H22 H23 F20 B/D/H/F Series Engines https://www.amazon.com/dp/B07L96MGJ6/ref=cm_sw_r_awdo_66W8SYQRH3SSRNGNG5ET