The only walk in spots in the valley are Camp 4, which is also a walk in campground and sort of a zoo - not a drive in and park campground where you'd take the family. I would shoot for something on Tioga Pass, or Bridalveil on Glacier Point Road. You want to get there in the morning, Tuolumne Meadows for example is half walk up campsites, and they fill by 10-11 am each day, according to the ranger at the gate.
the other thing you can try is going into the valley to the kiosk in the Curry parking lot and asking for cancellations - but then you would not be able to hustle fast enough to the abovementioned campsites to get a walk in.
If you are coming in on 140 there is a hostel called Yosemite Bug in Midpines, and a variety of hotels that may have cancellations as well. Also there is a campground just outside the park in NF on that route. Here is a map of NF campgrounds outside the park, just in case.
http://jrabold.net/yosemite/campn.shtml