Quote:
Originally posted by polarpi
In the chapter "Career Advice (# 29)" on the last page it's the twins' exit from Hogwarts - they say "If anyone fancies buying a Portable Swamp....come to number ninety-three, Diagon Alley - Weasleys' Wizard Wheezes"
So it's definitely in the book 
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The confusion may have come from the fact that earlier in OotP, before the start of the school year, the twins told Harry that they had not been able to find premises yet, so they were running the business purely as a mail-order business. They were running ads in The Daily Prophet, knowing their mother had stopped reading it because of the way it was smearing Harry.