I put this locket together for myself just shortly after the Kid's first hair cut.
You can order lockets like this all over the internet. (For example, here are some Etsy listings.) The little screw on top opens the whole thing up, and then you can just slip a snip of hair inside.
As those of you who are history buffs might be thinking, I was a little inspired by Victorian mourning lockets (wherein the lock of hair was from a beloved person who had died). But I think it's just as good for keeping a piece of a living loved one close to your heart. Particularly one who, his mama is afraid, will outgrow his blonde mop of hair all too soon!