Yup, generally speaking the mines. There's a range in which each of the dwarf scrolls are more common, but I don't know details. If I had to guess I'd say somewhere between L60 and L100 they're most common, but I have a feeling someone will correct me on that
1.5% drop rate for Scroll 3 as opposed to the usual .5% of other monsters.
resetting an infested floor in that 41-79 range should eventually get you what you need (and you'll probably pick up 2 in the process if you're at it long enough)
if i am recalling the wiki correctly the scroll breakdown is:
1 can be hoed up on any floor with dirt and is a monster drop from all monsters on floors 1-39)
2 can be hoed up on floors 1-39 and is a monster drop from 41-79
3 is a monster drop from 41-119, with the special bonus rate from blue/frost slimes
4 can be hoed up from 81-119 and is a Very Rare drop from all monsters everywhere
skeletons (71-79) do not drop scrolls of any kind, ever. don't waste your time on those floors if you're scroll farming.
I know a lot of players report struggling mightily to complete the set but I almost always manage to complete the set in time for the Dwarf's birthday (you need the translation guide before you can actually get any friendship points with the dwarf)