Single, white roses are usually wild species roses or first generation hybrids. For example, you could look at Rosa laevigata at HelpMeFind.com.
How big is the flower? Is it fragrant? For ID, it helps to have a closeup of a flat leaf (and size), closeup of thorns, and closeup of the receptacle (swelling under the flower or spent flower head). You can use the link box below the message box.
Here is a link that might be useful: Your first picture.