I'm a fan of this bib-collar blouse from Alice et June. I made a bib-collar from a uniform blouse that can be worn with either a white shirt or a contrasting shirt. Try look for Peter Pan collar shirt from thrift stores or ask your relatives if they have any outgrown uniform blouses. The fit doesn't matter as long as the collar is not too tight or loose on you. I would suggest Short Sleeve Peter Pan Blouse from French Toast since it's extremely cheap and the fabric is easy to care for. All you have to do is cut out the collar and trim the edge with trimmings.
Note: I ended up cutting a lot more than the center photo prior to adding the trims. I don't have any black buttons on hand, so I used black nail polish to paint the buttons black. I also added an extra button in the middle.