If you are looking for tax bills before Congress:
Congress.gov (Library of Congress’s federal legislative information site).
At the bottom of the “Bill Summary & Status Advanced Search Page” is a way to search for bills by using “Standard Subject Term.”
You may also find tax bills by going to the “Subject Terms Browse Page.” This page is a listing of all standard subject terms associated with bills in a given Congress. Click on the term and you will get a list of bills.