From the looks of it; plenty of reputable places to go for custom wheel builds.
I'll add another from personal experience/satisfaction - Custom Wheel Building

"keeping in mind my bike cost $600 so I'm really trying to stay at $400 or less"

Here's another perspective. You want to buy another set of wheels so you can swap between nice and
bad weather. US$400 can probably buy a used bike; similar if not better to what you have now. My
advice is to get another bike(used) instead. You swap bikes depending on the weather/conditions. I
would think swapping wheels would get tiresome, especially if you ride a lot.