If you like the nature, IVs, etc. of your Kirlia, you can get 200 BP at the Battle Maison in Kiloude City after defeating the Pokemon League and AZ and buy an Ability Capsule.
However, if you haven't beat the Pokemon League and are desperate for a Gardevoir with Trace, you must capture another Ralts at Route 4, or if you have beat the Pokemon League, you can catch another Kirlia at a Psychic Friend Safari (if you don't feel like getting 200 BP (plus each Pokemon caught in the Friend Safari has three random 31 IVS)).
Abilities are based off of personality values. Since you cannot see PVs and they are binary, there is a near 50/50 chance for getting the ability you want.
More Info on Personality Values