Be sure that your suspicions are justified. Even though an infidelity test is relatively inexpensive and easy to use, the consequences of using it could be explosive, especially if the test comes back negative and your significant other finds out you've been snooping. So, make sure your suspicions have a basis in reality before you buy the test.
Shop online. There are a number of websites devoted to selling infidelity tests. These stores will allow you to use a credit card to buy the test and will place a generic charge on your statement in case a spouse is looking at the credit card bills. One thing to look out for, though, is receiving a strange package in the mail that might arouse your spouse's suspicions.
Check out a spy store. Most well-stocked spy stores carry infidelity tests. Buying from a spy store will save you from receiving the test in the mail.
Decide what kind of test you want. There are a number of different kinds of infidelity tests, each of which works differently. Choose between destructive tests, which require that you damage the material or clothing you're testing, and non-destructive tests, which preserve the tested material. Also, you can use saliva and blood presence tests to supplement the evidence of a semen test.