You can find the DMGs or XIPs for Xcode and other development tools on https://developer.apple.com/download/more/ (requires Apple ID to login).
You must login to have a valid session before downloading anything below.
*(Newest on top. For each minor version (6.3, 5.1, etc.) only the latest revision is kept in the list.)
- Xcode 10
- Even Older Versions (unsupported for iTunes Connect)