For the most precise, run Google Adwords for that term, run it on a very low budget (but high enough that you get seen) and you'll find out how many people viewed your advert each day.
Alternatively, what I've found SEO companies do is to use this tool
which gives you Yahoo's searches for one month, and times it by 4, although as you'll see below, that could be rather conservative. It's not the best tool in the world, as it treats plurals and singulars as the same, as is apparent with your term which gives it a total of 157584 for "ipod accessory". Also, "accessory iPod" will be treated as a different term on many search engines as well.
To quote Hitwise:
78% of UK internet searches in the four weeks to 21st October 2006 were powered by Google, up 9% year-on-year. Yahoo! took the #2 rank based on share of executed UK searches with 7.7% of the search market. Together Google, Yahoo! Search, MSN Search and Ask.com powered 96.6% of all UK internet searches, up 2% year-on-year.
Hope that helps.