Do you already have a sharpener in your knife drawer? Lots of people don't even know they have one. It'll have a knife handle, but look like a round file. It also may not be as expensive as you think at a kitchen supply store, probably $10 or so for a cheap one. If that's not an option right now, a smooth flat stone about the size of your palm would work. Run the blade almost flat across the stone with pressure on the blade edge in long smooth strokes, being careful to keep your fingers and thumb well out of the way. It'll take several strokes on both sides, the more you do the sharper it gets, so it's a good kind of activity to do while you're watching tv or something. You could also use the sidewalk, but concrete can be a little too rough for a blade that thin, and you might chip it.