For my suggestion I'm going to assume the plane has a .40 or .46 two stroke engine. Get a second prop washer to make sure the nut does not bottom out on the threads before the prop is tight. Hopefully you can find one at a hobby shop because shipping would cost more than the part if you have to mail order it. Based on the .40-.46 assumption you need a washer for a 1/4" shaft. Here are links to several that would fit:
http://www2.towerhobbies.com/cgi-bin...?&I=LXEHG0&P=7 price $1.50
http://www2.towerhobbies.com/cgi-bin...?&I=LXEHG1&P=7 price 1.69
http://www2.towerhobbies.com/cgi-bin...?&I=LXEHG2&P=7 price 1.69
http://www2.towerhobbies.com/cgi-bin...?&I=LXCT70&P=7 price 1.89
http://www2.towerhobbies.com/cgi-bin...?&I=LXCT72&P=7 price 1.89

The list is not complete but it gives an idea of what to look for.