Check around at thrift stores or pawn shops for a bow, I would think.
For the quiver (the arrow holder), you could likely make a simple one yourself from paper mache, and then simply glue a fabric you like around the outside...sew it onto a strap to go around you, and you're good to go.
I'm an Archer, and a lot of us are actually very cool guys. Find an Archery Club, through your local Archery Shop. I'm sure somebody will be more than happy to let you borrow a recurve bow (not strung!) and a back quiver of cheapie arrows for Halloween.
For authenticity's sake, the bow in the picture is a three-piece Take-Down, possibly a Bear or a Ben Pearson, BTW.