Westlaw, Lexis, and Bloomberg have separate databases for just tax law.
Those three major publishers also have Tax Court opinions, but their coverage can be spotty for the unpublished/memoranda TC decisions.
You can also find Tax Court decisions on the Tax Court's website: ustaxcourt.gov
A refresher from the Main Sources of Tax Law page. Some big sources of tax jurisprudence are:
Abbreviations
AFTR: American Federal Tax Reports. They're published by RIA.
BTA Mem. Dec. OR BTAM: Board of Tax Appeals Memorandum Decisions
TC: Tax Court
TCM: Tax Court Memorandum Decisions
TCR: Tax Court Reporter. They're published by CCH or RIA.
USTC: United States Tax Cases
Special Citation Rules
Bluebook
ALWD
The Tax Code/Internal Revenue Code (IRC) is codified in Title 26 of the USC.
The IRS's own handy page is here.
Abbreviations
IRC or I.R.C.: Internal Revenue Code aka Tax Code
USC or U.S.C.: United States Code
Prior Versions
There are prior versions of the IRC, namely in 1938, 1954, and 1986. You'll see them referenced as 'IRC of 1954,' for example. If you just see IRC listed, assume it's the current version.
Special Citation Rules
Bluebook