What is the inkjet printable coating used CD-Rs and DVDs and where can
I buy the coating?

The coating is just an ultra thin plastic coating. 

CD screen printing is a technique where printing is done directly on
the CD surface using screen printing. This imprints the image
permanently on the surface. This is a very expensive option unless the
amount of CDs to be printed is very high. It requires a high degree of
precision on part of the printer and cannot be done easily unless you
have a screen printer.

Using blank CD labels for your CD labeling allows you to select the
kind of finish you would like to have for your labels. Screen printing
does not offer this option. With blank labels you can print your own
designs and logos on your desktop laser or inkjet printer and can
customize your label content on the fly. For most applications
printing on blank CD labels will prove to be much more in-expensive
compared to screen printing on the CDs. It will save a lot of time by
eliminating the need to co-ordinate with an outside printing source
for your blank CD printing. CD label applicators are available
commercially and cost next to nothing. By precisely positioning the
labels exactly at the center of the disc they make pasting the printed
CD labels onto the CD quick and easy and eliminate any air bubbles
between the label and the CD surface.