First of all, please keep in mind that being a moderator is a hard job – very much darned if you do, darned if you don't. Somebody will always be unhappy.
You can always post directly on meta if you have feedback, whether you're looking for an improvement or commending how something was handled. Please focus on the specific actions and behaviors though. Attacking somebody personally isn't constructive.
Some of us provide contact information directly on our user profiles. If a moderator voluntarily provides contact information, you are welcome to use that to provide them feedback. If a moderator chooses not to offer public contact information, do not attempt to find their personal contact information elsewhere.
You're also welcome to contact Stack Exchange with your comments. Use the Contact link at the bottom of any page.