Go to Great Outdoors and buy a load of badges & a necko & stich onto any outfit
or
Boy - go old style......pair of shorts, baseball cap with a badge stitched on the front, necko, t-shirt with badges & a necko
or
Girl - keep it simple with your fav dress & add a necko, beret with badge - easy to take off once the "novelty has passed.
either - treking shorts & t-shirt & stich on loads of scoutie things : tin cup, plate, tent pegs, etc....could be messy!!
You'll never buy a scouts/girl guides outfit in any hire shop.....but should be easy to put something togeather
PS - a week before the party, wear the clothes for a few days without washing, light a campfire & sit near it for a few hours, preferably in the rain & then store them in haversack (plastic bag will do). This will give any scout outfit a "real" feel & odour