Where can I buy black or white truffle mushroom?
@Cozmo has a range of speciality food and grocery requirements.
Where can I find sketch books for multiple art mediums?
@Virgin Megastore and @DNA Life Style - Abdali Mall has them in leagues
Where can I find a coffee shop with board games (something other than stories and local boardgames cafe)?
@Locals' (Turtle Green) is literally exactly what you're looking for
Where can I buy a decorative katana and tanto?
i think that an antique shop like @Atique Antique Shop would have them
Where can I buy thick leggings that I can wear alone suitable for winter?
Your best bet would @Mikiaj they the "Alsmah" brand
Where can I buy foldable flats (shoes for ladies)?
@Swefieh Veterinary Clinic if you go to Swefieh mall, you will find many many options for great prices!