In my experience, laser copy backgrounds are
laser copies of the painted background that
once went with the cel, and are usually copied
from the background that came with another
cel from the same sequence. For instance,
if you have a cel from a sequence, and 
another person has a cel from the same 
sequence with a background, they could make
a laser copy and give it to you (I imagine that
is how most laser copy backgrounds come 
about). If this is incorrect, please let me know.

-G.E.