Spring tensioner on letterbox flap?

My letter box is fitted to a Upvc door but the external flap tensioner has broken. (It consists of two plastc rods that slide into the top of the flap (one at each side both have a square at the far end) When fitting the tensioners you slighty twist them in on direction to gain the specified tension.
Do you know where I can obtain these rods as a spare part? Thanks.