If you're flying TG (or any *A carrier), *G entitiles you to access any lounge in those cities. I enjoyed the lounges even when flying domestically within Thailand on a cheapo fare.

As for finding the best fare, why not just buy a calling card (I use uniontelecard.com) and call the TG office in the country of departure (or Thailand)? It would cost less than $1USD (a few cents/min). You might try calling your local TG office first to get their price. (And write back to let us know which is cheaper.)