I need some way to make a letter and tear-off certificate out of a single sheet of paper?

I wanted to make all the children from our summer workshop a tiny certificate I could send them in the mail. I wanted it to be a normal piece of paper that you could like, fold twice to go.

The top two-thirds of the paper would be like a letter and the bottom third would be a little personalized certificate they could pull off and keep.

But I don't know where people get paper like that, or is there some kind of tool you could use to serrate the paper or something?

Thanks in advance for your answer.

It looks like a tiny pizza wheel, but it makes a cut then leaves a space. You can also get a cheap paper cutter with a wheel (not the guilotine type). They sell different wheels and one of them is a perforator. I always thought the perforator wheel would be a handy tool to have — but I'm such an office suply geek!