Amazon: I registered a new Amazon account using a relative's residential address in the US, and without registering a credit card. I was blocked from downloading certain free books because they detected that I was using an Australian server and wanted me to prove US residency.
Sony: won't allow me into their bookstore at all but not sure whether that's IP-based.
Kobo: uses IP address to determine your region.
I haven't tried B&N.
I have downloaded library books from overseas and using foreign servers. This MAY depend on your library but I haven't heard of anyone having problems. I am not sure if you will need to register for the service with your home library before you can download. You should also check what format your library has available - most use epub predominantly - which is incompatible with Kindle - but some also have mobi titles available, which are compatible with Kindle (but not the other major players eg Sony, Kobo, Nook).
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