Looking for a DNA sequence
There are several nucleotide sequence datbases available. One of the largest is the NCBI GenBank at http://www.ncbi.nlm.nih.gov/genbank/

You may use the search bar at the top to search for nucleotide sequences belonging to a certain organism or gene. For example, searching for "E coli" will give you the full genome sequences for several bacterial species, with a strain of E. coli as the top result. The E. coli genome is about 5 MB large. If you want a single gene sequence, searching for "sonic hedgehog" (without quotes) will return the human hedgehog gene as the top result, which is 1,481 basepairs long.

To get a sequence in raw format, select "FASTA" below the entry. At the page you get sent to  , chose "send" in the upper right corner and under "Choose destination", "file". This will give you a .fasta file, the standard raw sequence format. The first line, starting with ">" will be metadata, while the rest of the file consists of the nucleotide letters only.

Note that in addition to a,t,g and c, you may also find other characters, most often "n". These characters describe ambiguity in the sequence (meaning that the nucleotide at that position could not be determined). For a full description of the FASTA format sequence representation, see http://en.wikipedia.org/wiki/FASTA_format#Sequence_representation