Hello, welcome to the community and thanks for reaching out!
For offline installation of SonarLint Eclipse, you can use one of the zipped P2 repositories available on our binary distribution server. E.g you can download the latest (as of writing) ZIP for 5.3.0, then configure it in Help -> Install New Software -> Add -> Archive.
For VSCode, Visual Studio and IntelliJ, the marketplaces offer a link to download the plugin file (VSIX or ZIP) which you can then install with each editor’s extension management feature.