I'd say that b3 isn't much of a problem for black, especially if white plays like he did in your game. All he's done is help to open the diagonal, exposing his a1 rook. It didn't help his knight out much either.
Agree with Eladar, white's opening is nothing to fear. That said, 3...c5 looks good,
threatening to win a pawn. He can't play 4.d5 so he's forced to play something wimpy
like 4.Nf3. Only then play 4...Nc6, so you're not blocking your c-pawn as in the
game.
How to find this move? Basic opening principle: assault the center. Even if you play
something hypermodern like 1...g6, 2...Bg7, that shouldn't stop you from lashing out
if an opportunity presents itself.
Agree with Elmyr. ...c5 wants to be played eventually anyway if you can get it in that early with threats you now have won the initiative and played a nice pawn break.