Buying baby/toddler shoes-advice needed

I really want to buy some shoes for my son. Not crib shoes but early toddler shoes. I'm six months pregnant right now and I'm just not quite sure what size to buy. I really want to buy shoes that I see now but is it best to wait until he's at The age I want to buy the shoes for? I just don't want the shoes to go out of stock but I don't know what size he'll be when I need them. Should I just buy them anyway and put them on him when he can fit them?