You can find them at garden centers and online. Each type of soap is geared to control a specific insect (or insects) so be sure to read the label to make sure you’re selecting the right one.
Also, ALWAYS read the label directions and remember that more is NOT better! Use the product exactly as it is intended.
For information on the types of soaps available and how they work, here is a useful factsheet from Clemson University: Insecticidal Soaps for Garden Pest Control. Another factsheet from Mother Earth News is How to Use Insecticidal Soap in the Garden. Always choose a product that is organic; they will have the OMRI (Organic Materials Review Institute) seal on the label.