Hi there, first let me compliment you on your 
taste, I too am a fan of BP. I also have never 
seen any with a background from the series. This 
does not mean that i think the entire series 
backgrounds were done in CG. It could be that 
they were, but the backgrounds were drab to give 
the show the proper atmosphere, this may be why 
none have ever been seen if some exist in cel 
form.
Next i like your site, and your BP cel. I have 
one from that sequence as yours, but you may 
already know this that the cel you have is from 
the boogiepop at dawn commercial. 
At times i have seen lone backgrounds for sale 
and wasnt sure if there origin was from BP. If i 
come across any more like those in the future, 
i'll send you a note to ask your opinion, ok.
Take care, sorry i couldnt be more helpful.