A very often asked question and one that can take a little digging to answer. Here's a couple of good resources for California data which you can download via ftp then store and use offline for a SAR op. Download these PRIOR to needing the data, so you are familiar with the data you have and not waiting for lengthy downloads when time is of the essence.
Spend some time poking around these sites, you'll find lots of good info.
atlas.ca.gov Lot;s of good data here
CA 24k USGS basemaps
/pub/drg
Transportation and travel routes, water ways, imagery, NED, landcover etc.
/pub/casil
/pub/casil/imageryBaseMapsLandCover
nhdftp.usgs.gov
/DataSets
/hydrography
There's many more sources for data, these should provide you enough to get started.
tip: When displaying Tiff or SID files in ArcMap, change the display properties from Nearest Neighbor to Bilinear Interpolation for a sharper image.
