I am trying to install owncloud-client package on Debian 8 via apt-get install owncloud-client. However the package is only for the version 1.7 which does not let me log in my owncloud istance. Is there any way to install a more recent version?
Just wait until the opensuse build system has finished their maintenance work. This is out of the "hands" of the ownCloud community and there are no additional mirrors where you could download the software from.
In Debian Stable you can always use backports just adding the repos to your sources. These are update version recompiled packages maintained by Debian for the Stable version.
I recommend them to you, I cannot live without them in my Debian Stable desktop for some packages like LibreOffice, Mozilla Firefox Firefox, etc. and of course owncloud-client.