Big Flat Campground

First come, first served. There are only 9 campsites, but not busy campsite. Open June-October.
I went there Friday night of Labor Day weekend for backpaking that starts Saturday, but it was available.
Probably only backpackers use this campground.
The rest room is available with toilet paper. not stinky. water is not available.
Creek is nearby to filter water, but kind of far (it's under the hill). better to bring a big bottle of water for the first day of backpacking.

Quiet and a lot of tree. good for sleeping before starting backpacking.

Big Flat Campground is a hub for some backpacking trailheads including Union Lake and Caribou Lakes. Like Kaori said, the campground is first-come first serve, 2 vault toilets, no running water, and about 9 sites that are far apart and private. There's a public corral for horses too. The campground is right next to a huge, beautiful meadow and creek. We had no problem securing a campsite on 4th of July weekend. And it's federal land so you're technically allowed to set up camp wherever you want, so long as it's not on private property.

BE AWARE, Big Flat Campground is a LONG 40 minute drive down a dirt and gravel road. This was a little freaky and disconcerting at first in my tiny Hyundai Accent. Coffee Creek Road turns to gravel about 5 miles past the ranger station. Then it's another 15 on dirt and gravel. We did eventually make it to the campground and my little car was fine, even if we were driving incredibly slow on gut-wrenching cliff ledges. Just make sure you get plenty of gas.