anyone know a good place in Bangkok to change my old battery in my 3 year old iPhone 4S ?

TAN Telecom MBK. You can search the forum for details.

I did some resarch:

3 year old batteries after about 500 cycles can only hold less than 50% power. Even if the display shows 100%.

I want to order a cheap new battery online.

According to forums all the 'replacement batteries' from eBay or Amazon are fake (used).

However I found two reliable online shops for all kinds of Apple parts.

One in USA:

https://www.ifixit.com/Store/Parts/iPhone-4S-Replacement-Battery/IF115-005-2

one in England:

https://www.appleipodparts.com/iphone-4s-genuine-replacement-battery-fully-tested-p/hex09.htm

The shipping costs from Britain are more expensive.

I ordered from 'ifixit'

Why not just purchase a genuine Apple product. £59 for a fitted 4S replacement battery isn't bad and you know what you're getting.

I just recently replaced my battery on an iphone 5 at an iStudio repair center. 3500 Baht

I did ask at the JJ Mall a week prior and they were going to do it for 1000 Baht. However, it would not have been a "certified" battery. The risk of it causing damage or even worse, exploding in my hands was just not worth the price difference. Also, I noticed that the after market battery only had 1100 Amp. If I remember correctly, an original Apple battery usually has around 1900 amp.