Most bases are free, depending on which fleet you are obviously, due to a change in Aussie tax laws there are positions avail in most Aus bases, AKL is availiable as are most of europe except london cmds where all the old shool long sleved cpts reside waiting out there retirement.
You cant take a perm base as a s/o time till upgrade is 3 years at the moment may drop next year. Also to take certain bases ie AKL you must be relief qualified which happens approx two years after your upgrade.
The quickest way to get on a base is to join as a D/E freighter driver as they are all based.