Wanted to add...
This applies to all of Finland actually.
Shops called UFF
(has big yellow-ness around their stores
) and Fida
are both charity-based second-hand/thrift store chains, that organize&select their merchandise - so it's not just Random Crap(tm), but actually usable stuff.
They stock not just in clothes, but in everything from books, shoes, hats, scarves to misc. craft materials like fabric scraps, assorted bags-o-yarn etc. They're both found throughout Finland; esp. Fidas can nowadays be found in basically every town that has a population bigger than 10 000.
And also worth mentioning is that while UFF has a non-religious background, Fida in run by a lutheran (I think, correct me if I'm wrong here!) missionary organization.
Re: Helsinki activity, both chains are of course represented fairly actively throughout the city. For example, both have a shop on Iso Roobertinkatu, the pedestrian street in the center of Helsinki that's within a walking distance from both the bus station and the railway station.
Hope this helps anyone at all
& happy thrifting in HKI!