There are a few different sites you can use to generate new keywords for an SEM campaign. Here are some sites to use:
Google's Keyword Planner here. This is usually where to start, showing search volume of avg. cpc for specific keywords. You can also include the website to help generate additional keyword ideas.
SEM Rush here. Need a subscription for this one, but can be useful to gain insight into competitor keywords.
Keyword Toaster here. Usually use this to switch a list of keywords from BMM to exact so it is easier to upload into Google Ads Editor.