I don’t know the appropriate classical Latin for that, but in Neolatin we use Optimismi fautor or Melioris partis conspector or even In meliorem partem acceptor.
If you want to say ‘For optimists’ you have to put these in the dative case:
Optimismi fautoribus; Melioris partis conspectoribus; In meliorem partem acceptoribus.
This is all I found till now, I know it souds a little bit long, sorry.