I didn't end up going the powder coating route for the sliders I picked up, but may end up getting them done in the future. When I picked up the sliders from the Metal Tech, I asked the guys for a recommendation. They said they use PSP Powder Coating out in Saint Helens. I did give them a call and the price was really reasonable and they also said it would only take a couple of days.
Almost went with them, but ended up painting them myself so I could have them ready for a trip. Once I beat them up a bit, and pickup some skids I will probably get them all done at once.
You may want to pair up with some others, since a couple of places I called had minimums that sliders and skids alone would probably not exceed. Could save a bit.