I've tried some of the bigger BigCs (Rama IV), Tescos and HomePros. All sell them with cushion covers. Thanks in advance.

Try IKEA.

My wife buys her cushion covers here and there in Bangkok but the cushions (cushion fillers) separately at IKEA. The IKEA cushion filler sizes seem to fit standard Thai size cushion covers. I just checked in the IKEA Thailand on-line catalogue and cushions are listed. Follow the trail: Living Room/Textiles and Rugs/Cushions. I can see a 50x50 cushion filler (which would be about 20inx20in) listed there at THB 129. Sometimes difficult to find in the store when you visit, sometimes out of stock, but IKEA is definitely worth trying. Good luck!

Excellent. Hadn't thought of that! Thanks so much, Filer! I'm going soon!

Yup we also have them from Ikea but if you want real good ones then go to Muji and pay 900 a piece. They are made of special foam and much softer, the same foam is used in Auping bedmatresses, the top quality. They are sure worth the money.