Try google keyword tool analyser to pull off monthly estimations for keyword traffic. You can also use Google trends to get a visualisation of keyword traffic data over a period of up to a few years, this is pretty good to spot seasonal peaks and troughs for different phrases.
There are many sites out there offering information on certain trends. One good way to find keywords is to look at top sellers on ebay and amazon and see what people are buying, those are the words you want to target ones that make you hear the sound "cha-ching".