Maven repositories do provide simple way to download sources jar.
I will explain it using a demonstration for "spring-boot-actuator-autoconfigure".
Otherwise, you can always "git clone" the repo from github, if its there and get the specific code.
As explained by others, you can use "mvn dependency:sources" command the get and generate sources jar for the dependency you are using.
Note: Some dependencies will not have sources.jar, as those contains no source code but a pom file. e.g. spring-boot-starter-actuator. As in this case:
Starter POMs are a set of convenient dependency descriptors that you can include in your application. You get a one-stop-shop for all the Spring and related technology that you need, without having to hunt through sample code and copy paste loads of dependency descriptors.
Reference: Intro to Spring Boot Starters