The "Any CPU" nature of .NET means the same installer works on both x86 and x64. However, the runtime optimizes for 64-bit. Simply use the standard download.
Download Microsoft .NET Framework 3.5 Service pack 1 (Full Package) from Official Microsoft Download Center. Microsoft microsoft net framework 3.5 1 windows 7 64 bit download
Why .NET 3.5.1 still shows up