WEll, the first thing is to find out if your kids need testing. It depends on what state you live in. Some require annual testing, some semi-annual, some only in certain grades, and some have non-test alternatives. I live in Florida where testing is an option, one we don't choose. We pick the portfolio review option.
Check out
http://www.hslda.org for your state laws.
That said, most states require tests to be administered by someone other than the parent, usually a certified teacher. You may be able to have the kids go to a school to take a test, or the SAT if they are old enough. Some states require you to use a certain tests, some just require you to use a "standardized test".
There are test companies that sell standardized tests and will score them. There are also stand. tests sold by some curriculum suppliers. The problem is that many companies won't even consider selling you the test unless you are a certified teacher.
If testing is mandatory I suggest you find a large local or state-wide support group, ask the parents there where they get and have their tests administered.
As a last resort you can contact the local school dist. and try to have your child sit in on a test there (that's common practice here in FL).
In our local support group we have a few certified teachers who now homeschool. They are eager to administer tests and do our evals for about $35.
Take care,
Meg