I would never had imagined that people with smaller hands would have problems in Thailand, but comparing my hands with two of my Thai co-workers I found out that the size of the hands are not that different.
You can try Dirt Shop in Bangkok, I believe that they sell Alpinestars gloves... but I'm not actually sure if they will stock smaller than regular sizes – probably your problem could be even be bigger then shopping for big sizes.
Anyway if the Thai dealers not have the correct size or the right style of motorcycle gloves you can always look at Motorcycle Gloves.