Foxconn Technology, which assembles iPhones and iPads, has 200,000 workers in China already putting together new iPhones and making items such as metal casings, the Journal said.
Apple is expected to launch the new iPads at an event on Oct. 16.
Asian suppliers expect Apple's larger tablet to have a 12.9-inch liquid-crystal-display screen with a resolution similar to the iPad Air launched in October last year, the Journal said.
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Data research firm IDC said in August that it expected tablet sales to slow globally in 2014.
Apple was not immediately available for comment.
The company said it sold more than 10 million of its new iPhones in the first weekend they were available in September.