Find a site or two dedicated to folks in your market. These are usually forums/blogs etc. Since you say your core market is big, finding these should not be an issue.

Buy some ad on those sites that links/shows a poll "Do you own an Android?" or something to that nature.

That'll give you SOME idea. Also, don't forget that with a hot and new product like Android, the numbers are changing literally. Translation: don't depend too much on market size because it may literally change significantly month to month in Android's case.