if you put 'eggnog' into a search engine i.e google it will give pages for eggnog recipes for you to make your own, but i don't think they sell it over here.
As it seems to consist of a few very basic ingredients and is very simple to prepare, why would you want to pay over the odds for a ready-made mix? I can never understand people buying pancake mix, either - or instant mash!
Btw, after looking at the recipe, it's not far from Advocaat - If you go into an off licence and try it, you may be surprised. Or ask for a shot in a pub!