There are several good sources including the following:
WELD generates 30-meter composites of Landsat 7 Enhanced Thematic
Mapper Plus (ETM+) terrain corrected (Level 1T) mosaics at weekly,
monthly, seasonal and annual periods for the conterminous United
States (CONUS) and Alaska. These mosaics provide consistent data that
can be used to derive land cover as well as geo-physical and
biophysical products for regional assessment of surface dynamics and
to study Earth system functioning.
Earth Explorer, a familiar USGS site.
One of my favorite sites for Landsat data. I especially like the download interface.
GloVis, yet another familiar USGS site.
If everything continues to go well, Landsat 8 will be providing providing data to the public in the near future. From the USGS:
Data products from Landsat 8 will become available approximately 100
days after launch, and will be consistent with the existing standard
Level-1 (orthorectified) data products created using Landsat 1 to
Landsat 7 data.
The standard Level 1 data and LandsatLook (full-resolution jpg)
products will be available for download at no charge from GloVis,
EarthExplorer or the LandsatLook Viewer.
Details about processing levels of Landsat data products can be found