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90% don't drop they get auctioned off. Have a look at namejet.com and see if it's listed there. If it is and its listed as prerelease then you'll have to bid for it there. If it's listed as pending delete then it will drop and you'll need to bid on it at namejet and also:
snapnames.com
pool.com
You can also try the likes of godaddy etc. but if it drops chances are one of the 3 above will get it.
as Grant says - Anything decent in .com won't drop If it's with the likes of Godaddy etc. it's reregistered then transferred inhouse by the current registration provider. (usually 42 days after expiration)
You can get lucky with English words or spelling (if it does't fit the American Market) or it's with a smaller registration service that doesn't retain inhouse