Announced in February 2017, Google Custom Site Search is no longer being sold, nor will it be supported for those who do not have a current license. According to Fortune, ‘existing customers can keep using GSS for the life of their current license’. Multiple sources confirm that once a customer’s search allocations are exhausted, the account will convert to the company’s [Google’s] Custom Search Engine [CSE].
Google provides an ad-free version of CSE for nonprofits, schools and governments. This is a Google initiative and only applies to the Custom Search Element.
Other groups interested in using CSE can use the JSON API, which provides 100 search queries per day for free. See pricing and additional options.
In 2017, there was research into various options to the Google Custom Site Search by several companies. The most recent [crowdsourced and most recently updated in February of 2018] is provided by alternativeTo.net.
The site provides alternatives for all platforms [desktop, mobile, tablet, web/online, development, other platforms] with any license [free, commercial, open source].
Sorted by rank, by recent popularity or by recently added, alternatives include ‘likes’ and some helpful comments by users. Filtering by tags further narrows the search. A comments and activities sidebar provide further insight into recent updates.