Nasco sells a wide range of products from farm and ranch supplies, through arts and crafts materials and educational resources. They have specialized catalogs for the different areas, including science. But they also have an excellent website where you can find products from all their catalogs.
Their science website offers a huge selection of just about everything you could possibly need for science, and their prices are very reasonable.
For example, they sell a set of dissection specimens they call the Zoology Survey Set with Study Guides (#LS03737MH). It includes seven specimens and study/dissection guides. Add the Introductory Elementary Dissecting Kit (#SA01127M), and you've got all the basics for dissection lab work for a very reasonable price.