Taken from Macworld
http://www.macworld.com/article/1167855/installing_mountain_lion_what_you_need_to_know.html
People with slow or limited-bandwidth Internet connections: If your Internet connection is slow, it will take a long timeperhaps daysto download the 4GB+ Mountain Lion installer. Even worse, if your ISP enforces caps on your Internet-data usage, you could end up paying a hefty price for the privilege.
If youve got a Mac laptop, you can instead tote it to your favorite Apple retailer, the library, a friends house, or the officeanywhere with a fast Internet connectionand download the Mountain Lion installer there. In fact, when Lion was released, Apples official policy was to invite you to your local Apple Store and use the stores Internet connection to download Lion; store employees would even walk you through the purchase, download, and installation processes. I suspect that will continue to be the case with Mountain Lion.
Of course, if your Mac doesnt happen to be portable, or if you live in an area where you cant borrow a fast, cheap Internet connection, you'll need to find another solution. If youve got an Apple Store nearby, you may be able to take a portable hard drive or an 8GB-or-larger thumb drive to the store and ask to purchase and download the installer on one of the stores Macs; similarly, you could borrow a friends computer, orif youre lucky enough to have a Mac at the officeuse your work computer to download Mountain Lion.
Last year, Apple made available a $69 bootable flash drive containing the Lion installer; the company hasnt yet announced if it will do the same for Mountain Lion. Ill update this article with the latest information as it becomes available.
