You can get it online on sheetmusicplus, 8notes, sheetmusicdirect. You might also check your local library and local music stores. Keep in mind that the official title of the song is "Over the Rainbow," so searching for it under this title will yield the best results.
You can buy this online through Sheet Music Plus, JW Pepper, Hickey's Music Online, Pender's Music Company, Eble Music, Free Hand Music, and Mannerino's/Sheet Music Cincinnati. There are several versions available.
The company sheetmusicplus online sells the sheet music to "Somewhere Over the Rainbow," and they have an arrangement by Katherine McPhee for vocals and piano accompaniment. I'd really recommend this site due to its cheap prices. You can also try the regular sheet music stores such as Dale Music or the Music and Arts center.
