I found a way to get all the arrows you need
From Level 5 mid right of the map there is a 4x5 section of snaking pit traps. Drop down to "Level 6 trapped".
The room that you drop into is full of Skeletons. Don't kill them just dodge and make your way to the stairs exit and go up them.
Go back down the stairs but stay on them. Kill the Skeletons until the skelly archer approaches
He will keep shooting at you dropping arrows at your feet to pick up.
They are frost arrows too.