One downfall to the early pregnancy test is that if you have a negative, you may still be pregnant. This happens because the amount of hCG in your urine is simply not high enough to register as a positive. Or your pregnancy test could really be negative. If you have a negative result on an early pregnancy test, you should repeat the test after you have missed your period, counting your normal cycle length from the last menstrual period.
You can buy an early pregnancy test at most stores that sell pregnancy tests. You can also find them online with other pregnancy tests.
Obstetrics: Normal and Problem Pregnancies. Gabbe, S, Niebyl, J, Simpson, JL. Fifth Edition.