That sucks. Check JoAnne's, Fabric Depot, any other sewing places, and of course online. You might even be able to contact the place you bought you cosplay from and ask them to send you the button they used, so you get the exact one.
I'm defiantly gonna finish Pluto this year, but if I actually wear him at con...who knows...
But I know I wont have him for next year (well...maybe), so I'm trying to decide exactly which Noah's Circus character I want to be...