Maternity/nursing bras

I am currently unable to fit into my regular bras even though I am only 8 weeks along. When would be a good time to purchase maternity bras? Should I just buy a bigger size bra for now or buy a maternity bra for now? First time mom and only person I have to ask questions is all of you and my older sister. And she is trying to feed her two month old baby cereal already and never breast fed.