What is the purpose you need it for? Creating an install USB or using the disc image on a VM? Which type of installer image you want to use depends on that.
Here's a general purpose ISO that I found, haven't tested it though:
https://archive.org/details/mac-os-high-sierra-10.13.5
Apple also offers a direct download on their website still, but it requires a compatible Mac to download. If you've downloaded it in the past, it should still be available in your purchases.