As with all Envisat Data, Level 1 and Level 2 AATSR products should be obtained from ESA. ESA have provided a PDF document that explains the process in more detail, which can be downloaded from their website here . More details of AATSR Level 1 and Level 2 products are given here.
UK users can obtain AATSR data directly from the NEODC. Please click here for more details.
An alternate source of high resolution AATSR data, in near real time, is ESA's MEDSPIRATION project, which provides the 1-km SST data in GHRSST-PP Level 2P format, which is based on NetCDF. You will find that SST data from other sensors, notably AMSR-E, AVHRR and SEVIRI are also available in the L2P format, which makes inter-comparisons of different SST data sets particularly straightforward to do.