I am planning to buy I phone 4 and 3GS at Bangkok next week. Pl guide on the specific shop and price?

The prices change every few days, and is related to negotiation ability. Regardless, they are cheaper in the US. If you do not buy from True, you need to have the product jailbreaked. You can find them at MBK (BTS: Siam), and Pantip Plaza (BTS: Ratchathewi).