Stevia powders that don't contain dextrose include Sweetleaf Stevia, Planetary Herbals Stevia Powder, Nunaturals and Now Foods BetterStevia. Brands to avoid for their dextrose content specifically are Stevia in the Raw and Pure Via.
The aforementioned brands do not contain two other ingredients, maltodextrin, which is a potentially unhealthy, processed additive, and rebiana, which is a chemical substitute for Stevia. Other Stevia brands that do not contain dextrose specifically, but do contain these other potentially harmful ingredients, include Truvia and Pyure Stevia. Wholesome Stevia is another brand that does not contain dextrose specifically, but does contain agave nectar, an ingredient that has a high concentration of fructose that can similarly cause spikes in blood sugar.