This is the official guide for small businesses put out by the United States Department of Justice Civil Rights division. It provides a starting point for understanding requirements for small businesses.
This resource is produced by AccessibilitySwitchboard.org and provides guidance for small businesses and startups on creating an accessibility plan and handling accessibility related complaints.
This 2016 article from Small Business Trends summarizes the ADA legal requirements for small businesses
This article provides an introduction to accessibility for small businesses.