[mothered]

Whats the best way to get serial numbers?

If I may answer this,
Personally I go into shops and take a bunch of pictures of the boxes and the shop doesn't seem to care if you do this though know that the serial numbers for UE are not the ones on the box, they are on the base of the product. Another way is using nix.ru they have pics of the serials on there. If all these methods fail I normally find an unboxing on yt and just find a part where they show it then slow down to 0.25 speed then screenshot.

The simplicity of Googling the Item via Google Images, also provides results.

There may be the need to sift through quite a few pages, but I've obtained a lot of serials. Your method of taking photos In-store, Is certainly effective.