Thanks, Walter. Jcrayst, what I've found is that the "Frame, holder, IC, Assy" (this is Epson-speak for the metal and plastic box that has receptacles for the nine ink cartridges) for a 3880 will NOT accept the two magenta inks designed for a 3800. They will not go all the into the holder and seat properly. At least, they won't without modifying the holder somehow, which I preferred not to try. All other colors of ink, other than those two, are the same for both printers. As far as I know, the Ink System Assembly, except for the Holder, is otherwise the same for the 3800 and 3880, despite whatever differences there may be in the head, printer driver, firmware, etc. My holder for the 3800 seemed fine, and I doubt it would wear out normally. I had stored the printer after purging the ink and replacing it with Piezoflush, which kept all the ink pathways clean until I could re-install the parts.
I bought my Ink System Assembly from SDott.com in China, but their website is not working at the moment. Compass Micro still lists the 3880 version on their site, but the 3800 version is described as discontinued.