Last summer when my bottle of Happy ran out, I went to Dillards to purchase a new bottle. That was when I discovered Kenneth Cole Reaction (good suggestion, Goddess). It wasn't "similar" to Happy, really, but it is pretty much in the same category. Of course I ended up buying a new bottle of Happy just a few weeks later
Do try Banana Republic M & Banana Republic Classic. They're very citrusy.
And I agree w/ the Little Italy suggestion as well.
However, nothing will ever replace Happy to me. It was my first male-marketed scent that opened the door to men's frags to me.