Students in biology are often tasked with dissecting a frog, but there are many other vertebrate models that can provide a rich experience in learning anatomy. Common specimens include the fetal pig, shark, mink, cat, and the rat. Rats are fairly inexpensive and are raised for use in medical research. Rats are also larger than frogs which can make it easier to observe organs. You can purchase a rat anatomy kit from Amazon which will include the basic tools and guides needed to complete the investigation. In addition, you can download this rat dissection guide which provides instructions for dissecting the rat and identifying the major features. The guide begins with the external anatomy, continues to the muscles and skeleton, then instructs on how to cut the rat to reveal the internal structures of the digestive, circulatory, respiratory, and urogenital systems.
If dissection is not an option for you, you can simply view this video which examines the internal organs of the rat.