Ball bearings are cheap. You need to fill the cup and then remove 2 balls. Fill it up with grease, put it back together and tighten the top until there is no more play, but the bars still turn without resistance. THEN tighten the lock nut. You'll probably need to get a lockring wrench to hold the top in place while you tighten that.

If you want a bike shop to do it, it shouldn't cost too much. Where I work it probably wouldn't run more than $10