I am trying to find a good seamstress who would be able to alter my trousers around Sukhumvit. I have checked a few tailors but I am not willing to pay almost as much as I would pay for a new pair.

There are usually sidewalk-based tailors at the beginning of major Sois, or some short distance down them. You should be able to locate a few in a relatively short time. There are also job shops - shopfronts with dozens of machines, which perform out-sourced piece-work for the tailor shops - everywhere. Some specialize in pants, others in shirts and others in jackets.

The district along Soi Setthi Thawi Sap (between Suk 22 and 16) is where many of the rip-off tailors you're talking about send their clothes to be altered / manufactured. There's also a bunch of street seamstresses about.

If you get off the BTS at Phra Khanong and leave the station to Phra Khanong Post Office there is a seamstress at the bottom of the staircase.

If you get off the BTS at Phra Khanong and leave the station to Phra Khanong Post Office there is a seamstress at the bottom of the staircase.

Thank you for your tips. I have checked a few of the seamstresses which are on my street and they all rejected. They said to difficult. I have found one today inside of Times Square and will try them now as it is quite convenient and price with THB 300 seems to be ok.

If you haven't yet located a seamstress then try looking in front or the Bangkok Bank on Sukhumvit between Soi 8 and 10 or on Soi 4 behind the police box.

If you are still looking, Sukhumvit Soi 18 around 200 yards down on the left under a tree. She works with a cobbler.