HeinOnline has several collections relating to tax matters:
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Tax Notes Weekly
From the Westlaw homepage, in the "Browse" box, click on the "Practice Areas" tab and then the "Tax" link. This page compiles many types of resources (cases, statutes, regulations, secondary sources, and more) pertaining to tax topics.
Westlaw also provides access to Warren, Gorham & Lamont (WG&L) resources: treatises, manuals, and journals on tax. To find the WG&L titles, browse Secondary Sources and find the Warren, Gorham & Lamont link on the far right under Tools & Resources (or, alternatively search secondary sources for WG&L).