http://rc-car-online.de/shop/en/products/fg_8407_01-8407-01-square-wheel-driver-14-mm-steel-m6-2-pcs-.html
these work well and fit all wheels with squares.and they come in 7.5mm and 14mm
these are the spiders Ged mentioned (these fit particular rims, check which rims you have)
http://rc-car-online.de/shop/en/products/y0440-ht-y0440-rim-reinforcement-for-fg-1:6er-rim-6105-01-and-3105-2-pce.html