The short answer is that there isn’t one. If you’re van lifeing it quiet neighborhood streets are a good bet as long as don’t bunker in. Don’t bother with the canyons but there are quite a few boondocking sites along Peak to Peak north and south of Nederland.
Upper Dream Canyon is likely the closest legal camping area. It's free camping for 14 days I think, and there are NO fires. Only camping at designated sites too, I believe.
EDIT: Lots of construction in the canyon going on right now though.
Second upper dream, nice place to camp, and relatively close to boulder. More sport climbs in Boulder canyon than the flatirons, and upper dream is my favorite area in Boulder canyon, so many good lines in a pristine setting. Just make sure you are up and out by 9am, because 119 closes for blasting between 10am and 2pm on the weekdays.
Camping around nederland is also pretty abundant on forest roads, but at that point, you are pretty far out from the flatirons if you are hell-bent on sport climbing there, especially with all the construction delays driving back to boulder.