You can write your own spells. We have our brain for a reason. It's better to write your own spells. It's pretty easy to do especially if you're good at "writing", poetry etc etc. However,you don't -have to- use rhymes in your spells. When we make our own spells , it's our creation, the words flow from your heart, something which makes the spell ,even more "powerful", effective and important to you.