Before you start your search for public domain works that you can use to create products, you need to have a clear objective in mind. In other words, just knowing that you want to use a public domain work to create your own product isn't enough.
You need to know what type of work it is that you want to create, and what public domain work will fill your need. It is never a good idea to go into anything blindly. You will of course want to conduct market research, just as you would with any other type of product that you wanted to create.
Is there a market for your idea? A good way to find out if there is a market is to go to a search engine and type in keywords that would be used to search for your potential product. Pay close attention to the sponsored ads. Also go to your local bookstore and look at magazines that relate to your topic, again paying close attention to the ads. If you go to your public library, you can see back issues as well. Have certain advertisers purchased advertising space in previous issues as well? When dollars are being spent on advertising consistently, you can bet that there is a market.
Once you know there is a market for your idea, the next step is to determine how you want to deliver your product. Will it be a physical product such as a DVD, a CD, or a printed book or manual? Will it be a digital product, such as an E-Book?
You need to determine this, because the format that the public domain work you choose may matter a great deal to you. Now that you know what you want to create, and what format you want to deliver your product in to your customers, it will be easier to determine exactly what public domain material you are seeking – and of course knowing that will lead you to the right source to find it!