DVD+RWs allow random access, which means you should be able to add to them and erase from them without rerecording the whole disc. As I understand it, though I never have tried, DVD+RWs shouldn't finalize at all. You may have a stack of DVD+Rs that were mislabelled (it has been known to happen) .

Anyway, you can still get DVD-RWs from Amazon--and other online vendors as well.