NFL Team Payrolls from various sources. The "NFLPA Information Report, 2000" (thanks to Karl Einolf) and reports from the popular press found for a few years vary. The files are all in MSExcel format. So, use your "Save Target As" command to save them to your hard disk.
Combined League Level (various sources): 1970 through 2000
Congressional Hearings: 1952-1956
AP, SI, SBJ, BBWC, ESPN, NFLPA: 1991, 1997, 1999-2001, 2004.
USAToday.com: 2000-2007
NFLPA Information Report, 2000