Ishtar sources are available in a Git repository on Gitlab.
Ishtar version 4 has a Debian Bullseye package. Installation instructions are detailed on the page of the Debian repository hosted by Iggdrasil company.
Ishtar is a free software so you can obviously host your instance yourself.
If you don't have the required skills, and/or don't have a dedicated server and/or don't feel like managing that or if you would like to support the main developer, a hosting solution is proposed by Iggdrasil (small company founded by the lead developer of Ishtar and an archaeologist). Contact Étienne Loks by e-mail.