Primary legal sources are the law itself and emanate from the powers enumerated in the United States Constitution. Some researchers have heard the phrase Separation of Powers and they mistakenly search only for one branches documents but legal research is very much like a circle, you find one source, you may need to look to the other two branches to see how they have interpreted that source.
United States Constitution: https://www.archives.gov/founding-docs/constitution-transcript
United States Constitution including Amendments: https://www.govinfo.gov/content/pkg/CDOC-110hdoc50/pdf/CDOC-110hdoc50.pdf
United States Public Laws: https://www.govinfo.gov/app/collection/plaw (Can search back to 1995 - current to today)
United States Statutes at Large: https://www.govinfo.gov/app/collection/STATUTE (Can search back to 1951 - current to 2012)
United States Statutes at Large: https://www.loc.gov/law/help/statutes-at-large/ (Law Library of Congress has digitized the collection back to 1789)
United States Code: https://uscode.house.gov/browse.xhtml (Can search back to 1994 and is current to appx 10 days ago)
United States Courts: https://www.govinfo.gov/app/collection/uscourts (Can search 7th Circuit back to 2005 and District Court back to 2000 and is current to at least a month ago)
Supreme Court: https://www.supremecourt.gov/ (Can search back to 1991 and is current through 2018)
United States Regulations:
Code of Federal Regulations Annual Version: https://www.govinfo.gov/app/collection/cfr (Can search back to 1996 current to July 2019)
eCFR: https://gov.ecfr.io/cgi-bin/ECFR?page=browse (Current CFR to within a few days)
Federal Register: https://www.federalregister.gov/ (Can search back to 1994 and is current to today)
Low Cost Databases:
Fastcase is included in membership to the Illinois Bar Association: https://www.isba.org/fastcase
Versus Law: http://www.versuslaw.com/products/all.aspx
Casemaker: https://public.casemakerlegal.net/
Westlaw: https://legal.thomsonreuters.com/en/products/westlaw/westlaw-plans-pricing
Westlaw for small firms: https://legal.thomsonreuters.com/en/products/westlaw/small-law-firms
At the time of this guide I have not heard back from Bloomberg or Lexis about their low cost options but readers should follow up with them if they are interested.
Illinois General Assembly: http://www.ilga.gov/ (Can search Public Acts back to 1971 and is current) Note: There is also a link to Compiled Statutes but there is a notice that this is not an official source.
Illinois State Courts: http://www.illinoiscourts.gov/ (Can search back to 1996 and is current to today)
Illinois Code of Regulations: http://www.ilga.gov/commission/jcar/admincode/titles.html (Note this site says it is not official, that you need to contact the Secretary of State linked below, it also does not provide date ranges and when you click on Advanced Search it takes you back to the Legislature).
Illinois Register: https://www.cyberdriveillinois.com/departments/index/register/home.html (Note as above this site is not official, you need to contact the Secretary of State, it does however provide dates from 2002-current)
Illinois Secretary of State: https://www.cyberdriveillinois.com/