For a domain name, there are several options as you mentioned.
.in.th - This requires an individual ID. It is the simplest but unprofessional.
.co.th - This requires registration documents (Juristic person or
trademark) in Thailand or from overseas.  What documentation can you
provide?
.or.th - The requirement is similar to .co.th but a bit looser.
However, I don't think we (unregistered movement in infancy stage) are
qualified yet.
.THAI - It does not require anything but not available yet. No one can
provide a firm date that it will become available
However, all of these are equally difficult.  May I suggest
th.wikilovesmonuments.org instead? You could do this for other
countries too and redirect them to proper sites. (or vice versa)