Find real business or economic data recorded over time. The Statistical Abstract of the United States in the Mundt library is an excellent source of data. However, any appropriate source can be used.
Data can be yearly, monthly, daily, whatever.
The independent variable will be time and the dependent variable will be whatever you intend to forecast. There must be no gaps in the data. You may have to consult more than one edition of the Abstracts in order to fill in gaps. Each team must use data and tables different from other teams. Also, have your choice of data approved by the professor before doing the work to make sure you don't waste time.