Do they sell blue temporary hair dye? Is it expesive?

Probably, if not Walgreens does for sure, spray in, wash in, even chalks so you can do streaks

If not go to the beauty supply store. You can get the spray in or the temp application color.

I know they do normally i can't speak to their stock around Halloween, though. It is normally around 10$

Yes. And its usually not very expensive