How to get SKU Number

Might be answer is late but look at Simple Information of SKU (Stock keeping unit) number is, it's an unique tracking number (an arbi­trary num­ber) that are used in appStore for your application. You can put what­ever you want in there as long as it is unique among your appli­ca­tions. Try to fol­low a pat­tern for the SKU Num­ber of your apps so that you will be able to bet­ter orga­nize them. I sug­gest a com­bi­na­tion of the cur­rent year + month + ID for your app. So if you’re devel­op­ing your first appli­ca­tion on september 1991 (oh,, yah it's my b'day's month and year :D ), you could put your SKU Num­ber as “19910901”. Here, I am just suggesting you for this pattern but you can take/choose any pattern which easy for you.

SKU is short for Stock Keeping Unit.

It's a number you personally make up, which you yourself can use for keeping track of your applications. You can think of it as a product ID.

Source: What SKU Number to use when submitting apps?

Since there are already a couple answers on what it is, here's where to find it