Hi Lea Lea,
Most beauty supply stores will have a variety of Halloween themed fake hair for clip-ins.
I have also seen them on Amazon along with an assortment of different brightly color wigs.
I even saw some very cheap clip-ins at Sally Beauty recently in Halloween colors. You might just want to call around, or check Amazon, unless you want to try and color some blonde strands like Jamie suggested.
Good luck and please post photos for us all to see.