I would say you want someone to take on Fighting types. Also your team is slightly weak to psychic itself so having another resist isn't so bad.
After looking at the available option for psychic types I would say that Alakazam is your best bet.
Abra is available on Route 116, and in Granite Cave. It evolves into Kadabra at Level 16, then you simply have to trade it to someone to evolve it into Alakazam.
Now if trading is an issue for you than you can try out, then you can go for Gardevoir who is available as a Ralts on Route 102, and it doesn't matter what gender it is as Gallade isn't released until Gen 4