A swim cap

Does anyone know where to buy a large swim cap? I mean the kind not designed for tiny-headed masochistic bald people.

I've never seen one that would actually fit. When I try to put one on, it perches on top of my head like one of those tiny hats they had sometime in the thirties. When I try to forcefully pull it on it pulls my hair and hurts like a gas mask.

Also: do any of the swimming pools in Helsinki require them? I remember the time when Yrjönkatu swimming pool sort of did, or at least it said so somewhere, but nobody tried to enforce it.