Microsoft Publisher is the ‘desktop publishing’ part of MS Office. It has many formatting options not available in Word and is useful for making marketing documents like brochures, catalogs etc.
Publisher was somewhat forgotten because it was only in the high priced Microsoft Office bundles. That changed with arrival of Office 365 ‘subscriptions’.
Office 365 Personal and Home bundles include Publisher. If you have one of those ‘subscriptions’ then it will be installed on Windows desktop automatically. (There’s no Mac equivalent).
The cheapest way to get Publisher for a ‘one off’ job would be to get an Office 365 subscription trial for one month.
The next cheapest would be an Office 365 Personal subscription for one year at around US$70 (again, shop around).
You can buy Publisher separately as a ‘one-time’ or ‘perpetual’ licence. As a single program for US$110 or as part of Office 2016 Professional for US$400 (but shop around for discounted pricing).