Rick,
When I have taught others to scrimshaw on horn or ivory, I have them practice on horn or ivory. It is easy, and inexpensive, to prepare the horn surface but leave it thick. I have beginners cut straight lines, circles, letters, etc., to practice with technique and different tools. When done, scrape / polish off the designs and do it again. You can recut the same area many times and get a lot of practice out of one horn. It also gives you a lot of training in preparing a surface for cutting which is also an important part of the process.