If you don't have ay DME on hand is it ok to use dextrose as the sugar in a yeast starter?
The yeast will grow and multiply in dextrose. Supposedly, though, while they grow up in the dextrose they begin to down-regulate the genes required for fermentation of maltose. So when you pitch them into the maltose rich wort, they struggle a bit before fermentation really takes off. How big of an impact it has on the final beer is hard to say.
Furthermore, dextrose is a nutrient poor medium. DME is more than just a sugar source. It has nitrogen and other trace elements as well. So some yeast energizer would be needed as well in dextrose to hope to get much quality growth.
In an pinch without DME, some people have had luck purchasing a couple bottles of Malta-Goya from the local supermarket.