These seem to be called brake recessed nuts, rather then bolts - your online searches might be more productive if you change the search terms. That being said, the longest I could find in a quick search on Mordor.com (aka Amazon) was 30mm and up to 35mm elsewhere. Total cycling has a titanium 40mm for ~$8.50. After that, you might have to get creative - maybe buy two, cut the flanged end off one and join them together with a piece of M6 threaded rod - that way you could make the recessed nut as long as you like.