I just sharpen them myself with a sharpening steel that I got with my set of kitchen knives.
To be honest, I don't think you'll find anywhere that will sharpen them for you, as it's such a simple and menial task that people do themselves that it wouldn't be offered as a service and your question is like asking "Where do you get new shoelaces fitted to shoes?".
Sharpening knives for food preparation is very different from sharpening tools, so nowhere that offered a tool sharpening service would be suitable for knives, their tools and techniques would ruin the edge of a kitchen knife (as the angle of a kitchen knife blade and the cutting edge of a tool seldom the same).
You could get lucky and locate a specialised knife expert or tradesman who dealt in swords and blades, but even then I don't think there would be much money to be made in offering a sharpening service (or if there was you would be overcharged immensely).
The best bet is just to buy a good quality sharpening steel and doing it yourself, it's pretty simple but if you're unsure, there are plenty of guides to be found on the net, or you could ask butcher or chef to show you how they sharpen theirs (although some of the butcher's blades won't be the same as the blade on a kitchen knife).
Seeing as you're American, I bet someone in a gun shop/hunting supplies store could also show you how to sharpen up a knife really well, and while the angle of the edge of the blade on a kitchen knife and hunting/defence knife may not be the same, you could still get some good advice and techniques for sharpening your knives.