A state's tax administrator may go by various names, from Department of Revenue to Tax Commission, but they're the best place to find information on a state's budget and tax rate, and gather tax forms. See the Federation of Tax Administrators for a list, plus links to their websites.
Use a secondary source -- like a legal encyclopedia, treatise, ALR or periodical -- to find articles on your tax topic that cite to various states' laws. Secondary sources may be general or state-specific. Check out ScholarOneSearch for a research guide for your state; that will contain lists of recommended state-specific resources.
Where does each state get its revenue?
In addition to the state's tax administrator website, these provide some guidance:
To compare states' tax laws, try the following:
As Certain As Death: A Fifty-State Survey of State and Local Tax Laws by Susan Pace Hamill -- Call Number: KF6750 .H36 2007 (Law Stacks)
50 State Surveys: Tax Statutes - available on WestlawNext
Search your selected state(s) under the topic "Taxation" in West's KeyNumber System to find cases.
Get a list of tax statutes for each state using West's 50 State Surveys: Tax Statutes
For regulations, try RIA's State & Local Taxes - Regulations (available on WestlawNext) -- Select your state and search by keyword