Last time I was in Bangkok I could not find any 120 film (December 2006). I suggest you take film with you.
One Camera shop that might be worth trying is close to Khao San Road (well known traveller's haunt). I don't know the name, but I can give you directions.
If you go the end of Khao San Road where the Police station is (there is a big Wat - temple - on the other side of the road), turn left into Chak Kraphong Road and about 30 + meters on the left hand side, you will find the shop.
When I was there last time I was able to buy a red and an orange filter for my Konica Auto S3 (I had been looking every where in Bangkok for these!).
Good luck and enjoy the trip.
Andrew.