There doesn't appear to be a public Norman Colebourne collection anywhere.
I can only suggest contacting the National Museums Liverpool (or Walker and Lady Lever separately, maybe Williamson as well) and ask if they can help.
It might be worth keeping your eye out on local auctioneers (who have viewings) but it seems to have been very quiet in the last year.
Incredibly, originals have popped up on ebay in the past.
In a time of universal deceit - telling the truth is a revolutionary act. George Orwell
When the debate is lost, slander becomes the tool of the loser. Socrates