This is not a complete list, and I'll welcome suggestions for additional organizations to include. But these can get you started, and student memberships are usually so cheap that I'd encourage you to join multiple organizations while you can take advantage of student discounts.
American Association for Budget and Policy Analysis (AABPA)
American Society for Public Administration (ASPA)
Association for Budgeting and Financial Management (ABFM) of ASPA
Delaware Chapter of ASPA
Government Finance Officers Association (GFOA)
International City/County Management Association (ICMA)
National Association of State Budget Officers (NASBO)
National Tax Association (NTA)
Dr. W. Bartley Hildreth's links page is the bomb! I am very happy that he continues to maintain it. Without attempting to replicate that superb resource, I will post here a more limited selection of various resources I think might be useful to my students.
America's Founding Documents (National Archives)
Economic Policy Institute
Global Initiative for Fiscal Transparency (GIFT)
Institute on Taxation and Economic Policy
International Budget Partnership (IBP)
National Association of College and University Business Officers
National Association of State Procurement Officers
Peter G. Peterson Foundation Solutions Initiative
Public Budgeting and Finance News
The Tax Foundation provides a range of material, including this list of Ten Tax Myths Debunked
Tax Transparency video series by the International Budget Partnership
The Volcker Alliance, "founded by former Federal Reserve Board Chairman Paul A. Volcker, is a nonpartisan nonprofit organization dedicated to empowering the public sector workforce to solve the challenges facing our nation"
Committee for a Responsible Federal Budget - a scrupulously nonpartisan "deficit scold" organization
Congressional Budget Office - the nonpartisan scorekeeper in the federal-budget "game" and a source of high-quality analysis
Congressional Research Service Reports - excellent source of nonpartisan historical and procedural information and analysis (also see https://digital.library.unt.edu/explore/collections/CRSR/)
http://fiscaldata.treasury.gov/
Government Accountability Office - the referee in the federal budget game, responsible for financial and performance audits of the federal government and its agencies, among other things
House Appropriations Committee (also see the minority party's page) - major players in the game
House Budget Committee - major players in the game
House Ways and Means Committee - "All Bills for raising Revenue shall originate in the House of Representatives" (US Constitution Art. I, Sec. 7)
https://www.govtrack.us/ - I just discovered this. It looks interesting
Office of Management and Budget - a major player, whose URL as of 2025/02//12 redirects to a website that appears to feature only political advertising and no management and budget content (and https://www.whitehouse.gov/budget returns a 404 error). While we search for the useful information that used to be available from OMB, we can console ourselves with CRS Report RS61225 and archived material from the Biden, Trump 45, and Obama administrations
OpenOMB Apportionment Tracker (Protect Democracy Project) (source data from https://apportionment-public.max.gov/)
Senate Appropriations Committee - major players
Senate Budget Committee - major players
Tracking federal expenditures in real time (Brookings Hamilton Project, using data from https://fiscaldata.treasury.gov/)
Trump CEA Projections Tracker (Committee for a Responsible Federal Budget)
USA Facts - nonpartisan, independent source of curated fiscal and other information, including this excellent summary of US federal government revenues and expenditures