i got mine from www.dpsnutrition.com , they have one brand for the dextrose and that's NOW, and the maltodextrin is called Carbo Gain. Pretty cheap too
Try your local home-brew shop or the home-brew aisle of your local supermarket (dextrose is used in brewing beer ... part of the fermentation process I think).
Dunno if it is the same stuff but it is about 4 bucks a kilo so...
Dextrose is also used in sausage making. You can get it from on-line sausage making suppliers. Check with your local butcher shops. The home-brew supplies suggestion is an excellent one too. You will probably pay much more for the same thing at a health food store than at a brewers supply or sausage supplier.
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