Have you also considered using a skinny long bit holder such as the Milwaukee magnetic models (available in 150 and 300mm lengths)? You don't really get good quality extra long bits in sets, mainly I suppose because the cost would be in orbit. The longest good quality bits are generally 50mm long and can be useful when assembling or dismantling screwed timber frames where the screw heads have been set deep. When you do see long bit sets, such as Erbauer and other generics, the quality is simply atrocious. A few firms, such as Bosch, make 150mm bits and there are always the longer bits (up to about 170mm) made for various collated screwgun, such as the Makita and Hilti models, but you are generally restricted to PZD2, Ph2 and Sq2 sizes. Expect to pay £6 to £8 for a Makita long bit as fused in their collated screwguns - Hiltis, whilst better, are a lot pricier. None of the long bits I've tried give me anywhere near the length of life I can expect when using Wera Impaktor 25 or 50mm bits in a long bit holder