To scambait, you need to find active scammer numbers. There are multiple ways and sources to find those numbers and it also depends on which type of scam you’re looking for. Below are the most important sources.
PS: scammers are mostly active during US business hours and during weekdays. If you can’t find any active numbers this may be the reason.

Scammer.info

Scammer.info is our scambaiting forum with about 30,000 registered users. Founded in December 2016, it’s a general forum where you can find all sorts of scammer numbers, from tech support scammers that use fake popups to advertise their number over printer and router support numbers, IRS and SSA scammers to US-based scams and robocallers such as student loan and extended vehicle warranty robocallers. The forum allows users to collaborate and together investigate a specific call center and gather evidence which can also help to warn future victims because the forum posts show in the Google search results for the phone number. You can also use the forum to asks questions about scambaiting and find some tutorials with further information.

BobRTC

While BobRTC is mainly designed as a telephony tool to call scammers, it also works as a source of numbers. The built-in phonebook has a huge list of scammer numbers which are also sorted into various categories so you can find exactly the type of scam which you want to bait.

PopupDB

PopupDB was originally designed to automatically crawl the web for scam popups and then scrape them to extract information such as telephone number, URL, hosting provider, and a screenshot of the popup to create a database of fresh scammer numbers to scambait, to automatically report the popups to the responsible web hosting provider, domain registrar and telephone provider and also for evidence purposes, e.g. to support investigations of fraud cases by law enforcement agencies. Unfortunately, in 2021 the admin of PopupDB moved on from the project which meant that the database and scraper are no longer working. The generator does still work though: it’s a page that lets you choose via a slider how many browser tabs of sketchy links which may redirect to a popup you want to open. Be warned, it will also open NSFW content! This will often lead to a popup though. Since it wasn’t clear if PopupDB’s original generator will stay online, there is also an alternative, linked below.

Nomorobo / RoboKiller

Both Nomorobo and RoboKiller are apps that you can install on your phone to block spam/scam calls. On their website, they publish the list of numbers which they detected as scam/scam/robocall which we use to identify possible targets for scambaiting.