Wikipedia:Product placement

Policies, guidelines and process pages can serve as inadvertent product placement when they recommend specific commercial products. Such embedded marketing, even if well-meaning, is contrary to Wikipedia's prohibition on advertising and risks undermining the project's reputation for neutrality and commitment to the free content movement ("free" in the sense of intellectual freedom).

Where possible, instead of naming one specific product, project pages should provide or link to a fair comparison of available tools for a particular task. If there needs to be a single recommendation, prefer free tools where available and of comparable quality to their commercial equivalents. Avoid directing editors to products that may cause them harm or violate their privacy, such as companies known to engage in invasive tracking or harvesting of user data.

Examples
If it doesn't overburden the text, simply list alternative products:


 * Full-text books may be found on Google Books.
 * Full text book may be found on Project Gutenberg, Archive.org, or Google Books.

Otherwise, link to a list, on or off Wikipedia:


 * Before nominating an article for deletion, search for sources on Google.
 * Before nominating an article for deletion, use a web search engine to look for sources.