Stringsite.com's bass strings concist of a steel alloy plated with nickel. This method of manufacturing brings several advantages: The strings get a nice crisp and "round" sound as well as being more consistent and restistant to oxidation (rust) than 100% steel strings. Finally the nickel plating provides a softer feel to the strings which reduce finger noise (screech when moving across the strings) and also give longer life to both strings and frets.
The majority of strings sold today for electric guitar/bass are nickel plated steel strings - no matter what smart brand names are put on them.
Click "Single strings / Customize sets" if you wish to put together your own balanced sets or just need extra single strings.