I have something very specific in mind in terms of the dimensions, material and functionality of a sofa for my place. I need a good place where to get it done with good quality in Bangkok. Any recommendations?

Try any local furniture shop. Give them the design and they'll tell you what it'll cost.

I had good luck getting what I wanted at one of the frequent furniture shows at Sirikit Center. (Also at BITEC and Impact)

I picked a design that fit my needs then had it customized to my specs. Surprisingly inexpensive, (4 years ago)I ended up with a nice reclining sectional that works well as a guest bed for about B10,000

Don't forget to carefully measure your room and which side you want the "L". Especially important is to measure access doors and stairways/elevators. A friend didn't and his custom made sofa had to be cut up and reassembled; never quite right after.

Any recommended local furniture shops then? Location?