This one comes up a lot. There should be a FAQ.
The closest tent place is on the 91 at Featherly Regional Park.
But it is RIGHT on the 91. Bolsa Chica and Huntington Beach
State parks probably permit tents, but tenting on a paved
parking lot is not much fun.
There is one other spot, recently opened, which might be
OK. Just off the 71, off the 91
http://parks.ca.gov/default.asp?page_id=648
If you wanted to bend the rules a bit, you could, like
Jack Kerouac described in "The Dharma Bums", camp in
the area next to the Santa Ana River, in the reeds and
rushes. In fact that is just a mile or two upstream of
Featherly Regional Park. Also it is a few miles downstream
from the Chino Hills park mentioned above. There is an
alluvial fan from Coal Canyon Creek that makes a fine
camp spot; I've mentioned to the folks in charge this place
could be called "Kamp Kerouac". No luck on that yet.
There is excellent access -- on foot or by bicycle. I will
probably be there this coming Sunday morning.