DAISO: Japanese Paper in Bangkok
Don't forget the 2-dollar store
This is the place to find origami paper, if you don't want to cut it yourself. Each pack costs 60 baht, but the quality varies from kiddy grade (180 sheets) to real chiyogami in packs of 12 or 24. Mind you, only freaky folders pay 5 baht for a 15 cm square of paper.
Japanese paper in Thailand
You will find a selection of Awagami washi in branches of Somjai. This paper is expensive, but there are bumper sample packs for 250 baht. These contain assortments of scraps and and allsorts. Excellent value.
Curiously, much Japanese kozo originates in Thailand. Kozo is the Japanese name for mulberry กระดาษสา.