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Given the needs, I suspect you'll want to talk to one of the clickstream data providers about a custom deal. Jumpshot and SimilarWeb are the two I'd look at. Just be prepared to pay a lot, as their data is proprietary and they can thus charge quite a bit.

Thanks for you answers, I'll definitely be checking out Grepwords. I also want to look into the Adwords API in combination with excel, because these does seem to have an implementation, although I can hardly find any clear information on this. I just want to be able to paste a list of keywords and retrieve the search volumes for my country automatically.