I'd trap the dog, find it's owners and warn them, or get a shotgun. I may be young, but considering how many times I've lost chickens to a dog, I know that if you get
more chickens, you will lose that entire flock as well.
I am so sorry for your loss. I know how you feel, and I wish you didn't have to go through this. That's why you must take action
now! If you get more chickens, their whole lives depend entirely on
you! It is up to you to make the right decision. I would go with trapping the dog, since that gives him a chance. He doesn't realize he did anything wrong.
Once again, I am so very sorry for your loss. It must be a terribly hard time for you right now. I am sorry I cannot give you words of comfort, but I hope my words of advice will help.
*Lots of Internet hugs*