Z3ta+ is more of a virtual analog modular synth, esp. when it came out. It is pretty buzzy (rather than smooth if you want short hand) and wouldn't be the first place I'd look for pads. The same goes for Z3TA + 2, which is an update and has plenty of new patches and upgraded modulation. Z3ta should come with your version of SONAR. There are some libraries for it, but it comes with a 1000 patches.
You might have the Roland Groove Synth, an older synth that used to be included. Good stuff, including beatboxes, but there are plenty of fulsome synth patches too. You might have Dimensions Pro, which is exactly what you are looking for. A sample playback synth, there are plenty of string/pad patches and you can add your own samples - made or bought. Rapture is included with the top of the line SONAR, I believe, and it has been superseded by Rapture Pro. A really nice synth, combines the samples and libraries of DimPro and Rapture (they use the same engine) into a nice super synth. It comes as 3rd party and if you don't have the other 2 synths a nice buy - esp. if you wait for it to go on sale.
But you should have Rap and Dim light versions, which are mostly playback synths (not much access to programming). I've never checked but there should be pads there.
Check your version against the on-line Cake comparison chart.