In my previous post I said that this was from the Garden City Pottery Company. Well, I should have looked at the saucer and not the photograph of the saucer. The stamp on the bottom actually says Paden City Pottery Co.
After doing a little research I found that this pattern was called Rosalee and was discontinued in 1941. I found a source for replacements at Replacements.com. A cup and saucer set costs $17.99 and the cup I'm missing is $16.99.
Paden City was established in 1914 in Paden City, West Virginia and closed in 1963.