Privately held companies are not required to file financial disclosure documents with government regulators such as the SEC, so detailed financials usually are not readily available to the public.
Company profile sources may provide private company information such as sales, number of employees, number of stores if a retailer, and other general indicators. Journal and newspaper articles about a given company may give some general financials.
LexisNexis Academic , which can be also be accessed from the Business Portal  and the Rod Library home page , includes general information on private companies. After connecting, click the Business tab, then enter the private company name, such as Kwik Trip, and then click the Financial Information link on the resulting company profile.
Industry average sources, such as RMA Annual Statement Studies (UNI Reference HF 5681 B2 R6 ) provide median figures for companies in particular industries. For the convenience store industry, for example, median figures are provided for the sales/total assets ratio and other key income statement and balance sheet values.