Which test should I use and where do I obtain a copy of it?

According to the Georgia Home Education Association, there are five standardized tests that are most commonly used by homeschooling families. These are:

Iowa Test of Basic Skills (ITBS)
Stanford Achievement Test (SAT)
California Achievement Test (CAT) and (CAT/5)
Comprehensive Test of Basic Skills (CTBS)
Personalized Achievement Summary System (PASS)

The GHEA gives good, concise descriptions of each of these on their <a href="http://www.ghea.org/pages/testing/standardizedTests.php">website</a>. They also provide a list of testing suppliers which can be found by clicking <a href="http://www.ghea.org/pages/testing/satSuppliers.php">this link</a>.