They are stored in the same place :) You need to add the repositories separately in the settings.gradle file like so:

pluginManagement {
    repositories {
        mavenCentral()
        gradlePluginPortal()
        maven {
            url 'https://repo.spring.io/milestone'
        }
    }
}

Full example (which builds for me): https://github.com/willb611/hello-world-spring-boot-gradle/pull/1/files

Edit: if in doubt, you can find list of versions available at different repositories (i.e. spring milestone repository) here: https://mvnrepository.com/artifact/org.springframework.boot/spring-boot?repo=springio-milestone