SDKs (Software Development Kits) are bundles that contain headers, binaries, and other tools needed to build for a specific version of iOS. These can be found within Xcode at
Retrieving without a Mac
The legal way to download an SDK without access to a Mac is to download a copy of Xcode from Apple. These are available at developer.apple.com (free developer registration required). Usually the latest SDK is best, but if you want an older version, find the Xcode version containing it from this table.
A shareware program, TransMac, can extract Xcode DMGs on Windows. Open the DMG in the program, then use the left directory tree to navigate to
xcode.dmg/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs. Right-click the SDK directory and choose "Copy To".
Alternatively, grab a copy from here.
Expansion needed: How can modern Xcode .xip releases be extracted on Windows/Linux?
As of iOS 8, some people found that those instructions didn't work with the latest version of TransMac (version 11), but version 10.6 may work.