How can I retrieve my product key without the disc or case?
Your Macbook Pro doesn't have a product number; it has a serial number.

You can obtain your serial number by looking at the bottom of your Macbook Pro or by clicking  the "apple" at the top of the screen and selecting About This Mac



Once to select about this Mac, you will get a screen similar to the ones below that will give you your serial number.

For Mavericks or earlier....



For Yosemite and later....