I haven't come across too many of those cels that 
are for sale. If you look at any of the Japanese 
sites, make sure you search for 'Go-lion' (Lion 
Voltron) and 'Dairugger X' (Vehicle Voltron). I 
think that Southern Cross was actually pieced 
together from the anime 'Mospedia.' Transformers 
should still be Transformers.

You might try Mangaya from time to time. They've 
had Dairugger cels in the past.
http://www.solointer.com/mangaya/cel/cel.html

Hope that helps.