Hello. I can not tell you the exact length of the cables, since they're nicely tucked in my PC, but roughly from memory and eye measurement, they're around 100cm each, which is MORE than enough for any build. And as far as I know, you should have no problems when plugging in 2 extension cables one after the other.
As for buying the extension cables seperately, I did a good amount of searching, and, for the life of me, I could not find them, so unless you have more luck than me, you're going to have to make do with what you get.
Hope this helps :)