Luna_slave is correct. What people here in the States know as Robotech is really three different unrelated shows produced in Japan and "repackaged" over here. The original names of these Japanese shows are Super Dimensional Fortress Macross, Super Dimensional Calvary Southern Cross and Genesis Climber Mospeada. Harmony Gold acquired the rights to each of these three entirely seperate shows and dubbed a brand new English storyline on top of them that unified the shows as three chapters of one story. The reason why they did this is because none of the original Japanese shows had enough episodes to qualify for a sindicated time slot on American TV, while all three combined did.
So anyways, back to cel collecting. If you are interested in cels from the Macross Saga portion of Robotech you can find them listed as Macross on YJ and private dealer sites (though they will be mixed in with Macross movie cels as well). As for the other two chapters, Masters and New Generation, good luck, they are quite rare. Search for Southern Cross to look for cels from the Masters Robotech chapter. These are by far the most rare of any of the "Robotech" cels. One just happened to be put up on YJ today under the Macross heading. There is a Southern Cross heading on YJ but currently there are no cels there. There is one other on a private site that I can think of but because of the rarity is very pricey.
http://homepage.mac.com/clodjee/Catalog/Cels.htm
To find New Generation cels, search the links we have already provided, or YJ, but look for the Japanese title, Mospeada. There are a few cel there at least.
I hope this helps get you underway.