I understand what you mean by finding it difficult in making your own copywork. It should be a simple task, but it never worked out so well for me. Here are some sites I have used for copywork.
I think the one I’m most consistent with is the book, Spelling Wisdom, from http://simplycharlottemason.com. I don’t have a reason, but it’s the one I like and use.
Others that I have tried: George Washington’s Rules of Civility, 124 Quotes for Kids at www.oneshetwoshe.com, Quotes from Shakespeare’s Plays at Living Book Curriculum (may have paid for that or it was included with something, not sure anymore), Copywork Quotes from: The American Revolution and Copywork Quotes from: The Snow Queen both free at www.harringtonharmonies.com (she has several other free copywork pages), Dragons of the Bible free at homeschoolcopywork.com she also has a membership site for additional copywork. Fall Copywork at simblissitycottage.com, and classicalcopywork.com has free weekly copywork sent to your email and paid copywork from several sources such as Aesop, Bible, poetry, and McGuffey’s Readers. Hope this helps!