Nitrosamines, as far I know, come primarily from eating an amine-based food (citrus, raisins, cheese, fish, tomato, etc.) and a nitrate laden food (lettuce, spinach, beets, etc.). Now you can get some from processed meats (which also have nitrites) - they add it as a preservative - and from smoking. (Tap water will also often have lots of nitrates.)
Dr. Bryan argues that nitrosamine in whole foods cannot be an issue. His reasoning is that there is zero evidence that eating nitrates causes cancer - in fact, quite the opposite. And the reason may also be that flavonols and Vitamin C inhibit nitrosamines. http://link.springer.com/article/10.1007/BF02855742
"Nitrite scavenging effect by flavonoids and its structure-effect relationship"
So there are likely many compounds in good, whole foods that protect us from the nitrosamines.
I think the confusion regarding creatine is the fact that there is such a thing as creatine nitrate and the nitrate is broken off, from what i understand, and thus available potentially for combining with an amine. However, most creatine is creatine monohydrate I believe, but the bottom line is that it depends which form you purchased.