Anyone know a website where I can get a Katniss costume for Halloween

No, but you can get a Mockingjay pin (I don't know where they sell them, but you can get the pendant and attach a safety pin or something), a fake bow and arrow sheath (or a real one) and dress in whatever you think she wears. And braid your hair.

just wear old jeans and a t-shirt and rub dirt on your face. and braid your hair. no need to buy one.