Today, Microsoft released new ISO images for Windows 10 19H1, or more specifically, build 18290. The news comes after there were no ISOs available for the last week, being that build 18272 was removed last Tuesday.
The reason for this is because older builds will be expiring in a few days, so it didn't make sense to continue offering them. It was originally said that new ISOs wouldn't arrive until there was a new build in the Slow ring, but it turns out that build 18290 isn't ready for that yet, and these are sort of extenuating circumstances.
Windows 10 build 18290 was originally released to the Fast ring on November 28, and it's since been replaced by build 18298. While you can now do a clean installation of build 18290, you'll probably end up in the Fast ring and end up upgrading to build 18298. The Slow ring shouldn't be an option, being that the Slow ring is actually still on build 17763, the RTM build for the October 2018 Update.
If you want to download the ISO images for Windows 10 Insider Preview build 18290, you can find them here.