I get almost all my concentrates from chefs vapour now, their range is really good in my opinion.
As others have said for 'complex' flavour concentrates the chefs own range, pink spot, mt baker vapor, T juice are all good. I've tried several from each except T juice where I've only tried red astaire, and not had a bad one yet from these brands.
(Still on chefs vapour) Mom and Pop's concentrates have some good ones as well although from my experience they are a bit more hit and miss. Same goes for OneOnOne flavours I think.
Other brands that generally make concentrates for making your own recipes (e.g. TFA, Capella) do have a few that would work well on their own.
As for other sources, I've used LT ecigs for a few concentrates. Good service from those guys and generally their flavours were good (1 wasn't) although their concentrates are more single flavour types.