Here is the 'find an IBCLC' section of the IBCLC Examiners website:
http://americas.iblce.org/find-an-ibclc

My LO was 5 and a half weeks before he regained his birth weight. However, we had trouble BFing at first and he lost 1lb2oz in the first 2 weeks. After the second week his weight gain was fine, but it took him a while to 'catch up.'