Hello, my name is Fabio Fabbri, I live in Udon and I am new in the group. I have one request: do any of you know normal and used stores selling manga in Thailand? Preferably Isan and Bangkok, but any other city is equally well. Thank you so much for those who help me.

At the very bottom of the Adunlayadet Road (the Nong Sim end) there is a shop called Japan.
It is next to the two large internet games shops.
I have never been in it, but they are probably a good place to start.

They will of course be written in Thai. If your looking for other languages that may be difficult to find in Udon.
If you read Thai, most of the local book stores have a comic book section. Try B2S bookstore in central for Thai, and AsiaBooks on the bottom floor for English books (they can order books for you there also).