IBM used to have their own ISP, which I used for a few years when working in Africa and other places. Very handy. They stopped that a while back, which is a shame. The only other option for truly global use is, unfortunately, AOL, who have a presence in most countries. If you just need it for a few weeks, you could get a trial account, but be prepared for hassle when you want to cancel. They say that they don't require credit card details, but they will then require your billing phone number, so they can bill you if you don't cancel.
Alternatively, there are quite a few countries that don't have the free local calls that the us has, and they have free ISP usage, for the cost of a local call. This is what I do in the UK, when I'm there. You might try doing a search for a turkish option, if that's where you're going to be.
Cheers,
J.