Indeed. The best target to train your assassins are other assassins. For some odd reason they always have a 90something success rating. You need a spy to spot these guys though.
If you are playing an initially rich faction in vanilla, (like the seleucids for example) diplomats can also save you alot of trouble, because the (captain-led) armies of your enemies will usually be dirt cheap. So for the first couple of years during your buildup play Rome:Total Bribery...