Most Federal tax forms are posted on the the IRS Web site.
Most state tax forms are posted online. You can find the online state forms using FindLaw's Tax Forms page.
State and Federal Forms are also available from Lexis, Superform (back to 1993), Checkpoint (e-Forms), BNA ("interactive forms") and CCH's Perform Plus. In addition, Superform subscribers can order tax forms back to 1970 from the Superform "tax form hotline" (800-541-7197). They have just about everything.
Alternatively, you can try to get copies of forms from tax libraries, or you can call or visit an office of the relevant taxing authority.
If you truly can't find a Federal form anywhere else, try the IRS library (202-622-8050).
For more on state tax forms, see the entries for individual states.
IRS Tax Form Coordinating Committee: The Tax Form Coordinating Committee designs and modifies IRS tax forms. You can get minutes and agendas for their meetings by writing to the IRS Office of Disclosure. Be sure to reference the name of the relevant form and year(s) in question.