You can only get it in 16 GB. I'd advise to pick up a microSD card to expand your storage space. To install it, you just remove the back case to the phone and it slides in. I bought a 64 GB one and it's been working great.
SanDisk - Pixtor 16GB microSDHC UHS-I Memory Card
(2,463)
SanDisk - Ultra 32GB microSDHC Class 10 Memory Card
(2,084)
SanDisk - Ultra 64GB microSDXC Class 10 Memory Card
(978)