Tenchi's one of the cels that all of the dealers 
have -- it's like a universal series because of 
the wide release of the cels. 

In the first page of keep track on cels.org, the 
cel dealers should have at least one cel of Ryoko 
for sale. 

On the third page of dealers, try Ichiban, Art 
Rose (Washu and Ryoko), Anime Galaxy (Ryoko), 
although this is admitedly not a comprehensive 
list. 

Here are some places you should keep an eye on in 
the future: Miki's Jungle, because when she comes 
back from Japan, she'll probably bring a few with 
her. Noriko's Anime cels, on page three, said she 
was going to update with Tenchi cels as well 
soon. Also keep an eye on anime-cel.com, because 
they often have huge Tenchi updates with a large 
variety of cels for sale. Yahoo and ebay at times 
have more Ryoko cels than you can shake a stick 
at.  

Hope you find something you like.