Every so often, Robert Crumb and Alexander Wood work on a beautiful Etching to put up for sale - available here on rcrumb.com. While the stock may be limited, and options as well - the quality is top tier & the rarity is what makes these etchings so special.
In addition to etchings, rcrumb.com has a variety of comic books, serigraph prints, and giclee prints made from Robert Crumb's artwork - popular pieces and the lesser-known, you'll find no shortage here.