We have a decathlon near us in the Midlands, and always buy the kids helmets from there. Decent ones for about £10. Bought myself one this year for £13, worth a look if you have one near you!!
Jason, everything is more expensive in Bulgaria!!!! Buy them at home. I brought 11 kids out last week and was able to supply them all with new Tresspass helmets at £20 each from a local shop. Helmets were £15 to hire for a week in Pamps!!!