So you're going to be a spy, then?

Worse comes to worse, use eyebrow pencil very carefully and detailed-ly drawn on. Is that a word? Anyhow, you can get a fairly decent looking mustache that way.

I can have a fairly decent looking mustache now just by waiting a week so I don't need the fake one anymore. I only needed it because I had just shaved a day or so before hearing about the party.

You didn't say if you went or not, or what you went as. Care to tell? :)

even a week would be small time to have a good grown moustache

The hard way to do it is to get a tiny crochet hook, some fine netting, some fake or real hair (you can buy switches of it), and some spirit gum. Pin the netting to something as close to a wig block as you can get it. (Draw a light outline of the mustache shape you want on a piece of paper for under the netting.) Take a few strands of hair, loop it into the netting in the middle, and pull the long ends through, and pull it fairly tight, without tugging on the netting. Do it again. And again. Until you have your mustache haired. (remember that hair doesn't grow in a straight line, really. Look at your pattern of hair growth for a guide.) Cut it the way you want it. Spirit gum it to your upper lip.

That's the way they make wigs, too.

The easy way is the spirit gum the ends of hair onto your upper lip, but it's messy and some people have a bit of a reaction to the gum. It's also harder to get off.