The easiest thing to do when you need to purchase blank discs is to get them directly from Apple, which ensures compatibility and has always had good pricing.
But if you want to get blank DVDS on your own, make sure that you're purchasing DVD-R General media. If the product packaging or salesperson says that the disc is DVD-R but there's no indication of whether or not it's General, chances are that you're fine. You'll occasionally come across DVD-R Authoring media, which won't work in the SuperDrive.
Another thing to look out for if you're shopping for blank discs is that you're purchasing DVD-R (minus R) media and not DVD+R (plus R) or DVD+RW (plus RW) discs. The plus discs are designed for other kinds of DVD burners.
To get a better sense of things, glance through Table 18.2, which gives a good indication of the situation consumers face as a result of the Format Wars. (It's sort of like the VHS versus Betamax competition when VCRs first came out. But, in a nutshell , DVD-R is better and more compatible with DVD players, and that's what you have in the Mac, so get DVD-R media.)
Table 18.2. DVD Recordable Media