ikea, they have those flannel fabric for as low as $3-$5 https://www.ikea.com/sg/en/p/vitmossa-throw-grey-70304890/
They now have those pop up stores in mall selling blankets and bedsheets for just $10.
Shopee and Qoo10 have it too or Pasar malam nowadays have it everywhere
I bought it from china town market. It is quite good.
Taobao. This kind 150cmx200cm cost $6plus..
Carousell? I bought online at lazada
I buy from those shops at HDB blocks
IKEA is good for this type of stuff
Buy from market
Ezbuy or q010