Bloomberg Tax
Bloomberg Tax has a wonderful state taxation section. You'll see both state and multistate resources, tax forms, sales & use tax rate finders. (Insert heart eyes emoji here.)
They also have state chart builders to compare, say, Alabama versus Wyomings's corporate income tax group taxation topics. If you're wading into state taxation waters, this is definitely worth a look.
Lexis
In addition to its federal materials, Lexis has a pretty good list of state materials (like state Board of Tax Appeals decisions).
Westlaw
Westlaw's state taxation section is also pretty solid.
Secondary Sources
Did you see the previous mentions of UST's Library Search? Here it comes again - USE LIBRARY SEARCH!! You can find some awesome and weird and very specific materials there.
*TIP: search the state name and tax. The examples below resulted from typing Minnesota tax in the LibrarySearch search bar.
Some examples:
The Minnesota Department of Taxation: An Administrative History
Do Normative Appeals Affect Tax Compliance? Evidence from a Controlled Experiment in Minnesota