Where can I find these frequencies for several species?
Since most of the genomes consist of double helix DNA, the only meaningful ratio is GC-content. A comprehensive list of the known genomes that includes GC-content data is available at NCBI (note that default Overview tab doesn't contain the GC-content numbers, you need to select another tab, e.g. Prokaryotes). They also provide an FTP access to the raw data, if you need to process the numbers with software.  

If you're interested in particular genomes that are not present in this list, you can calculate all frequencies from the sequence. There are a number of programs to do this. For sequences in FASTA format you even don't need a special software, the analysis can be performed with standard tools, e.g. grep (should be installed in Linux/BSD/MacOSX, obtainable for other systems):

grep -v '^&gt;' sequence.fasta | grep -o '[ATGCatgc]' &gt; tmp
# number of AT pairs    
grep -c '[ATat]' tmp
# number of GC pairs
grep -c '[GCgc]' tmp