Students are often finding it difficult to locate GRE material on the web and ultimately head to the bookstore to purchase an expensive test prep book (that's what I did anyway...). Fortunately there are a number of free (and official) test prep solutions out there for both the GRE and the Physics subject test.
Here is a good list to get started (more info available from the SciEng Library website)
EST Registration page
You may register for a GRE test here.
To prepare, download the PDF of the GRE Test Prep Booklet
GRE Powerprep® Software (free) can be downloaded to your desktop and includes the computer version of the GRE practice test as two computer-based GRE General Tests.
Of course, now you need some places to apply:
Graduate School Directory for Physics and Astronomy
by the American Institute of Physics (AIP). A comprehensive online source for researching graduate programs in the physical sciences, Astronomy and related fields.
The following I found in our library catalog (http://mncat.lib.umn.edu/) using the keyterms "Graduate record examination"
Title: Nova’s GRE prep course [electronic resource] / Jeff Kolby, Scott Thornburg.
Published: Los Angeles, CA : Nova Press, c2004.
Title: Peterson’s interactive GRE test prep [computerfile]
Published [Princeton] : Peterson’s, c1997.
Availability TC Learning Resource Center Digital Collection LB2367.4 .P48x 1997 CD-R185 One Week Loan
Title: GRE : graduate record examination general test / Thomas H. Martinson.
Published New York : Arco : Distributed by Prentice Hall Trade Sales, c1990.
Availability TC Walter Sci/Eng Library Books (Level F) Quarto LB2367.4 .M374 1990 Regular Loan