How far along were u when u found out the sex?

I'm going tonight to find out the sex the place I'm going to tells me that they shouldn't have a problem finding it as long as the babies legs are open. I'm 16 weeks 3 days is there a good chance they will be able to tell me the sex or is it a waste of money to go now? Just really want to know the sex so I can start some shopping on items that will take a while to get here.