A few years ago a friend of mine had the same issue- he brought his suit to a place that does shoe repair, clothing alterations and luggage repair. They had found the zipper that was needed and fixed it for really cheap.
walkingon has the right advice. You can also try those tailors at a local mall to do it while you shop or whatever. Dry cleaner places can probably help you too. If it's just a zipper tab that needs resetting, I can't see that costing more than a few bucks. Worth it if rest of suit is all good. Let us know what you end up doing and where you went for repairs please.