1. Proverbs and sayings. What can you come up with, say, to fit the proverb "love is blind"? A book of proverbs is great for dipping into for themes you'd like to try to write to and generally are not that expensive to buy.
2. Classic fairytales. Look for the themes behind them. A common one is that of wrongs being righted (see Cinderella, Snow White, Hansel and Gretel to name but a few). How could you use that theme? And that is just one to play with! There are others.
3. Films. Again look at the theme but also look at the theme of the sub-plots (there will be at least one in any good movie).
4. Advertising slogans. Don't copy word for word but adapt. (This ties in nicely with my CFT post this week on Adaptations!). For example, in the UK, there was a slogan from years ago which was "go to work on an egg", advertising the virtues of eating eggs. Your theme could be something like "go to work on..." and name a foodstuff of your choice or a vehicle we don't see on Earth etc. Let your imagination run riot!