Is it possible to buy a locked iphone 4S from the US and use it in asia locked?

The only way that you could use a locked iPhone on another carrier network would be if you were roaming internationally. The cost of doing that would make the unsubsidized cost of an unlocked phone direct from Apple look dirt cheap. In most of Asia you'd pay $3 per minute for local calls as they would be routed as international calls. Text messages to your local friends would cost you 50 cents each. You'd also pay several dollars per megabyte of data. The phone bill of the typical teen would run several thousand dollars per month.

You could not use a locked iPhone on your local network since it would be locked to the US carrier only. There's no current unlock procedure for a locked iPhone 4S.

Additionally the cost of a subsidized phone would be roughly equal to the unsubsidized cost once the contract termination charge was added on. You must commit to a 2 year contract in the US to get the unsubsidized pricing. A non US resident would have to put up a cash deposit or a credit card hold at least equal to the termination fee plus 2 or 3 months billing just to get the phone since they have no US credit rating nor US income to pay the bills. The total cost to get the phone would be higher than the cost of an unlocked unsubsidized phone directly from Apple.

Subsidized 32 GB iPhone 4S from AT&T: $199
Contract termination fee: $399
3 months advance bills: $360
Total to get phone: $958

Unsubsidized unlocked 32 GB iPhone 4S from Apple: $649 (Available in late November)

I don't think anyone will sell a locked iPhone to a traveller. But if your friend manages to get one you can unlock it through third party unlockers like Mobileunlocksolutions.com and remove the network restriction permanently. After which you can use the iPhone 4S with a local sim. You can look in online for the price of iPhone in Singapore.