NBPN = no beer purchase necessary and it refers to mail-in rebates put out by beer companies like Bud, Miller, Heineken, Corona, etc. In many states, the beer companies can't require the beer purchase but still offer the rebate. So you will see a rebate that's like, "Save $5 when you buy $10 worth of meat and two six packs of Miller Lite." But the tiny fine print will say "NBPN in these states." If your state is listed, then you can do the rebate without purchasing the beer and still get the money back! So you'd just buy $10 of meat and send the form and your receipt for the meat in and they send you $5 back in the mail! Amazing!!!! And there are a ton of them out there.
As for where to get them, they are often in the beer aisles of grocery stores (since they are put out by the beer companies). Personally I don't have much luck finding them, so I trade for them here. Some folks (lucky duckies) live in places that get a lot of rebates so they trade them. Hope that helps!