Yes, parts are easily available, but not in bulk quantities. PakSuzuki keeps the bulk of their spares at Karachi and the service centres keep a minimal count of spares. If a certain part is out of stock, they order it from Karachi. Usually it takes one or two working day(s) to get parts from Karachi. So, it often depends on the user's luck.
I have used my GS150 for over 20,000 kms but haven't changed the cam chain, drive chain/sprocket set or rear tyre. The cam chain tension required adjustment at around 16,000kms or 18,000kms, easy, external procedure. I have noticed that the drive chain requires almost twice as much cleaning and oiling compared to most Pakistani bikes with fully enclosed drive chain casings.