Although some websites claim you can obtain free PSN codes, they are never free. Most sites ask the user to complete a survey or complete an offer for a product in order to receive a code generator. The problem is that there are no PSN code generators that are legitimate.
Surveys and offers are created to make money, but the money goes back to the owner of the website rather than the person who thinks they are getting the program. Other methods include websites using a system of points that relies on online shopping, watching adverts, Internet searches, among other options. The problem with this is the extensive amount of time a user must expend in order to be awarded a "free" redeemable PSN code.