There are sites where you can hire costumes like this, but to be honest it is easy to do for yourself. Get a pair of school short trousers from somewhere cheap like Asda or Primark (they probably won't be used again so no point in spending a fortune.) a short sleeved shirt (plain not patterned) and preferably a hand knitted woollen jumper, which will add realism to the overall effect, if you can get some beige socks that would be good and school shoes will probably be ok, just not trainers, plimsolls would be ok if the school shoes are not right. If you want to see more, see if you can Google pictures of 'Goodnight Mr Tom' or pictures of 1940's evacuee children, that should help.