Heart models can be useful for learning the structure of the heart, particularly when paired with a dissection. They can be purchased at a reasonable cost at Amazon, usually mounted in a way that you can see the external and internal anatomy.
When doing the dissection of a sheep heart, refer to models of the human heart to make comparisons. One major difference is the number of vessels that branch from the aorta. In humans, the three vessels are the brachiocephalic, left common carotid and left subclavian. On the sheep heart, you will likely only be able to find the brachiocephalic (depending on how the heart was cut.)