The best download link is the Mac App Store. Save up for the $199 license, or try the 90-day free trial of the current version. You might find that the modern Logic handles your old projects even better than 10.3.3 ever did.
I scanned the DMG with both Malwarebytes and ClamXAV before opening – results were clean. If you download this, always verify the file size (approx. 1.2GB) and checksum if provided. For peace of mind, install on a separate test volume first. Logic Pro X 10.3.3 DMG Download