I haven't messed around much with crafting random scrolls (I did it once hoping for a master geomancy scroll and got bless), but I was able to craft the skillbook by finding a scroll for it on a merchant (in my case, the enchantress in Cyseal square).

I'd look around at all the magically inclined vendors you have access to. You can also get lucky with a chest/barrel/etc, that's how I got the scroll to craft my Earthquake book.