I know starbucks do lovely drinking chocolate and when i was in Ny last Xmas they were selling all their brands of coffee and chocolate in big sachets. Not sure id they might have what your looking for but heres their website anyway Starbucks.com
Otherwise have you tried butlers?They might sell their own brand or if not point you in the right direction.
Why not buy a gorgeous container, a big bar of good 70% chocolate and some golden demerara sugar? Then grate the (cold) chocolate preferably in a food processor, mix with an equal quantity of sugar and package in your lovely container. This is all it is - if you buy good ingredients you get good results!
Green & Blacks is lovely, pretty sure it's 70% and is available here in Habitat, Nourish etc. I usually get it in Sainsbury's up t'North. It's v chocolatey and dissolves nicely too.