Everyone knows how that episode went right? They went to Rarity who was in the Boutique for some reason, and I vaguely remember Twilight being mobbed by Rarity's hospitality. I also vaguely remember them being in City Hall where the main decorations are supposed to be.
So I made up an excuse for them to go in a Boutique, Rarity was simply taking a lunch break, and Rainbow Dash simply directed them straight to Rarity.