I am looking for rockwool, net pots, nutrients, and expanded clay growing medium.

There used to be one but it was shutdown a while back. The only hardware store I know that might have planting equipment is the K-rauta in Ruoholahti.

Isn't there a thread where people post the most cost effective sources for nutrients, lights and other grow gear?

Otherwise, what is the forum's favorite online grow store?

The first that springs to mind is Chilitarvike.fi, which is run by our very own Kärsä Babar. Chilikauppa.fi also have various growing supplies and is run by our gracious host Janne. Both are online shops with fast service and in my experience quite sensible shipping costs.

I'm afraid I can't help you with local shops, as I don't live even in the capital region.

These are the ones I know of:

Puutarhurinverstas (http://www.puutarhurinverstas.com/catalog/), Kruununhaka
Viherpeukku (http://viherpeukku.fi/), Kallio
Elm Trading (http://www.pispalanpalmu.fi/shop/), Kallio
Herbert (http://herbert.fi/), Malminkartano

Yet another online store with supplies: http://www.jolokia.fi/