You can find both at Fairprice, probaby cheapest there too.
I think you be able to get these at any supermarkets
you will get flax seeds in fairprice as well!
You can find it in NTUC or any supermarket.
I bought my organic flax seeds at ntuc .
NTUC should have these ingredients! :)
You can find brewers yeast in NTUC
Try online stores!
You can try iHerb