Earlier this year, I saw a whole buncha Oniisama E 
cels cycle through ebay (I THINK it was ebay).  
They were all pretty cheap, I judged, given that 
they looked pretty old and all.  I didn't buy any 
because I didn't recognize the series, and I 
didn't like the style enough to buy it just 'cuz.  
So anyways, they DO exist, and they're not even 
very expensive (but they might be hard to find...) 
Sorry I can't help you anymore than that! ^_^;;