It is now possible to purchase individual spare parts for several of the Thule roof bar fitting kits. It maybe you just need a replacement part rather than a complete fitting kit. The list is updated on a regular basis and covers several popular kits with more being added as soon as they become available.
To see if spares are available for your fitting kit simply select your fitting kit number from the drop down menu. Unfortunately if your kit number is not listed it means that spares parts for that kit are not available.