In the Sheung Wan Civic Center you'll find the large wet market (between Morrison St. and Bonham Strand, main entrance off of Queen's Rd. Central). On Queen's Rd. at Possession St. there's also a good stand selling most veggies, and the one on Queen's Rd. at Morrison St. is pretty popular, too.
The Fusion on Queen's Rd. (near Possession St./Bonham Strand) in the Fook Sing Court is well stocked (even for expats with some organic options).
For house cleaning products, there is Shui Shing Hong soap shop at 130a Jervois St. http://www.shiu-shing.com.hk/ (It looks like it's more an essential oil shop, but they have good quality liquid body soap, laundry detergent, cleaning supplies, and you can return the bottles to be reused!)
For stationary, there is a shop on Queen's Rd. Central between Morrison St. and Cleverly St. ("north" or "sea" side of street). If you walk further towards Central on Queen's Rd. there's also another stationary shop past the Wellcome/Hollywood Terrace but before Wellington St. (on the "south" or "hill" side of Queen's Rd.). I like this shop, they're very friendly and relaxed. Not too crowded.
There's actually a lot of book stores/stationary shops with kids stuff at the "Li Sing Primary School" bus stop in Sai Ying Pun on Pok Fu Lam Rd. You can take the 37A, 71, 7, 4, 4X, 55, 90B, 91 minibus from Queen's Rd. in Sheung Wan to this bus stop. It's just a few stops away. And if you walk up a bit on Pok Fu Lam Rd. there, there actually is like an architecture supply store, which would have more serious art things, I think.
Hope that helps!