The ad rates for print magazines and newspapers are typically easy to find. Many publications will post their advertising rates on their websites. Simply search for "media kit" or "advertising rate" and the name of the publication.
If it's not listed on their website, you can contact the advertising department of that publication to get a quote on the type of ad you're considering.
The ad rates for TV and radio can be more difficult to find. The rate usually depends on a variety of variables (e.g. time of day, length of ad, number of repetitions, size of audience, etc) and therefore is typically negotiated with the station or media company.
The ad rate for online advertising can vary by platform and type of ad. Use these resources to search for the average ad rate for the major players (e.g. GoogleAds, Amazon, Instagram, etc.) and use this to estimate your own costs.