Note: My answer contains minor spoilers for the runes necessary to cast fireburst (the very first spell scroll you find), fire shield, ice shield, and also the locations of the runes for the four elements (fire, ice, air, and earth).
There isn't a scroll for that spell. Seeing as you don't require the scroll of a spell to cast that spell, it's safe to assume you're intended to experiment with spell casting.
Think of it this way, the first spell scroll you find explains how to cast the basic fire spell, and thus you learn that the 7 rune (look at your numpad) is the fire rune. If you reach a point in your fire magic skill tree (we'll say 16) and you see that you've unlocked the ability to cast a new spell but you haven't found the scroll for it yet, you should definitely start experimenting by selecting the fire rune (7) and trying other runes in combination with it to see if you can discover what it is, because just like in this scenario there very well may not be a scroll for it. Just try different combinations: 7 and 8, 7 and 9, etc. Eventually you would come to the conclusion that 7 and 5 results in the fire shield spell and that you could just as easily make an ice shield with 3 and 5 instead because 3 is the ice rune (the four elements are in the corners).
If you need further confirmation that you are by no means cheating by doing this, think of it also from a lore perspective; wizards are scholarly, they're eager to gain more knowledge of their arts and they are no doubt constantly studying and experimenting with their spells, trying to learn new ones and to make their old ones better. You are without a doubt intended to experiment with your spells/runes this way, if you weren't the developers would have made it so that you also require the scroll of a spell in addendum to the necessary skill level to cast it.