The 16-gigabyte version of Google\'s Nexus 7 tablet disappeared from the online Play store during the weekend due to overwhelming demand, online reports said. Anonymous sources \"close to Google\" told the British newspaper The Guardian the U.S. search giant had \"seriously underestimated\" demand for the top-of-the-line model, priced at $250. The $200 Nexus 7 with 8 gigabytes of memory is available only through Google Play and is currently shipping within three to five days, PC World reported Monday. The tablets are proving hard to find in retail outlets, with Staples listing them as available in-store only and on special order, and GameStop is advising customers the 16 gigabyte version is back-ordered. Google said all existing orders would be shipped by Friday, while a message on Google Play urges potential tablet buyers to sign up for e-mail alerts to learn when the tablet will be back in stock, PC World reported.