I got mine from a thrift store. The first was a newer fill-in-the-blank book. Our current one is an old school copy, from back when workbooks were not to be written in, lol.
Anyhow, it's this series here
Scott, Foresman and Company D'Nealian Handwriting
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There are printables online that you can make or just print out if the book prices become too high. I really like this set, though. Each day has 2-4 letter or stroke practice and an activity. Towards the end they get creative practice with writing poems, letters, invitations, applications..ds likes the series a lot.