There are several sites where you can get bearings at good prices. http://simplybearings.co.uk/shop/ is one example, look on Ebay or try TheMZShop.co.uk. You need a pair of 6306 bearings with C3 fit.
However, before rushing into stripping the engine you might want to describe the nature of the noises, MZs are not the quietest engines mechanically. A lot of the noise comes from the clutch and primary drive area. Does the noise abate at all when you operate the clutch?