Exchange 2019 CU1 was recently released as part of the February 2019 patch Tuesday updates. This was originally discussed here on the blog. Exchange 2019 is intended for large enterprise customers, and as such is distributed through the Volume License Centre.
Exchange 2019 CU1 is now available for evaluation on the Visual Studio download centre.
In my Visual Studio this is file:
mu_exchange_server_2019_cumulative_update_1_x64_dvd_6c695b62.iso which is 4.8 GB.