Many Clarion units have the code printed on:

Searching for a can be tricky because many results that claim to be "free" are actually paid services or third-party apps with mixed reputations. Genuine code generation usually requires specific serial number data that manufacturers or authorized services often charge for. Where to Find the Code for Free

However, if your vehicle is a 2015 model or newer, the free generators likely won't crack the encryption. In that case, look for a reputable eBay seller who asks for your serial number and charges less than $10 for a guaranteed code.