Since you're apparently flying directly to Chengdu, you might want to consider airlines that offer through fares from the US (CA, MU, and soon UA). Otherwise, you'd be burning money if you didn't use a Shanghai-based TA. I posted my favorite TA's (Liz) phone number in the Shanghai thread in this forum. Several others have successfully used her since.

Purchasing and picking up the ticket might be a little challenging if you're not planning a Shanghai stop.... but, not impossible. Since your flight isn't until December, you could pay with a credit card (4% surcharge, but still much less than US booking engines) and have your ticket sent to the airport or mailed to you. Alternatively, if you know anyone that's going to SH between now and then, they could buy you a ticket.

I've never booked a ticket more than 3 or 4 days in advance so I'm not sure if the deep discounts get loaded months out (though I'm guessing they do). In any case, what you're looking for is a 40, 45, or 50% discount. Any TA can book these. If they're not available when you call, ask when they think seats will open up.