Yep, luna_slave is correct. The best place to get Macross/Robotech cels seems to be Yahoo Japan. There are a number of them up there right now including a couple of multiple cel lots. There are also several private dealers who sell Macross/Robotech cels (Anime Link having the best selection but also higher prices in my opinion). Check out some of the sites on this keeptrack site and you will be able to find Macross sections on many of them.
http://mise.x25.se/pub/keeptrack/
As for pricing, unfortunately Macross is one of the higher priced anime out there. For cels of main characters from the movie, Do You Remember Love, expect to pay upwards of $300. Cels from the Macross TV show (Robotech) can generally be bought for perhaps 50% less. As luna_art points out, nice cels of Minmei are particularly popular (especially among deep pocketed Japanese collectors) and can regularly draw $500-$1000 for the top quality ones. Cels of Millia are somewhat rare and have been drawing similar prices lately.
So I guess my advice is to either:
1. Save your pennies, because chances are you are going to need quite a few of them.
2. Be patient and shop around. There are nice cels out there at good prices. Also, the same cel at auction can usually be had at a lower price than if it was on a dealer's site.
3. Focus on collecting other, generally less expensive anime. I loathe this option myself but it does make sense for monitary reasons.
Just a few of my personal observations being in the Macross cel market for the past 2 years.
