All campsites in LPNF are first-come, first-serve, but even on the weekends (barring that it's not a holiday weekend), there are plenty of awesome backcountry campsites. Willett Camp is located nearest to Willett hot spring and is about 7 miles away from the Piedra Blanca trailhead.
Another five miles down Sespe River Trail is Sespe Hot Spring Camp and Sespe Hot Spring Palm Camp. Both of which are great overnight options near Sespe hot springs. Keep an eye out for bighorn sheep! They like to hang out around Sespe hot spring.
The name Sespe can be traced back to a Chumash Indian village, called Cepsey, Sek-pe or S'eqpe' ("Kneecap") in the Chumash language
Sespe Wilderness is also home to the Sespe Condor Sanctuary. It's not open to the public, but it was established to protect one of the most endangered animals in the world. What's a major threat to condors? Microtrash (i.e. bottle caps, broken glass).
California has the second largest number of wilderness areas in the country (Alaska has the most)