I have 2 tikkas that have to be ten years old and after replacing the straps they are like new again.
http://www.allbatteries.co.uk/lighting/accessories.html
they are genuine petzl and although the picture in the link isn't clear it's a direct replacement for many of petzl lamps. in fact the kit included an over the top of the head strap too if you like those type. Good price, good comms too with text/email order updates.
Word of advice though, carefully study the routing of the old strap before removing it, it isn't that clear once you take the old one off! I had 2 to do so had one left to refer to.
Highly recommended
