If you wanted to this analysis for a large number of gene symbols and/or from the command line, I would first download gene_info.gz from here, and then use awk to parse. For example, SELL has the Entrez Gene ID of 6402, so:
gzip -cd gene_info.gz | awk '$2==6402{print $5}'
produces this output:
CD62L|LAM1|LECAM1|LEU8|LNHR|LSEL|LYAM1|PLNHR|TQ1
(The second column of gene_info is Entrez Gene ID, the fifth column has the aliases)
You can also do a similar awk parsing based on the gene symbol directly, but then you probably also want to limit it by organism (e.g., human=9606). For example:
gzip -cd gene_info.gz | awk '$3=="SELL"&&$1==9606{print $5}'
produces the same output as above...
To get a file that translates all human gene symbols to their aliases:
gzip -cd gene_info.gz | awk '$1==9606{print $3"\t"$5}' > output.txt