You can buy it DEF Walmart or at any automotive part store. You can also get it for a reasonable price at a VW dealer. BMW dealers usually sell at more than twice the price as anywhere else.
You have to be careful at not overfilling because it can damage the DEF tank sensors and clog the tank vent system. I usually add 2.5 gals to the passive tank at one time. If I remember correctly, the filler neck for the passive tank is the lowest one.
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