Alita cels are very popular lately -- when they 
are on an update, they get snatched up very 
quickly. It doesn't help that there were only two 
OAV episodes, so the supply is relatively low. Be 
prepared to pay quite a bit of money for them 
too.  
I think My Dizzy Anime World has two of Alita 
that he's going to sell on the next update. You 
can find his site on on Cel Dealers 3 on 
cels.org. The cel dealer update feature is a 
great tool, if you're looking for different 
series.  
Good luck.