Try a brewing supply store, although I doubt that it would be very cheap. You could also try chemical supply companies. There are also food grade ingredient suppliers who carry it in bulk. You could probably buy it in 50 lb. bags or truckloads.
What is your use for the dextrose? The reason that I ask is that it is really just another simple sugar, a lot like cane or beet sugar, sucrose, sold in grocery stores. I can't remember the chemistry of the sucrose or dextrose, but they are normally measured by a sweetness index with sucrose being the standard, and the sweetest of the two, IIRC.
If you are just looking for a cheap sweetner, stick with sugar. You really won't find anything cheaper that's nearly as palatable or readily available. I haven't bought dextrose for a long time, but the price that you are talking sounds very high. Domestic sucrose runs about $21 in bulk and world sucrose runs about $.14. Note:
Link to futures. The near term contract is usually close to the spot market price.Go to page 6.
The only consumer use for dextrose that I'm aware of is as a cheap sweetner to add when making beer. The yeast consumer the dextrose & break it down, leaving alcohol. In other words, it's just a cheap kick. Add too much, though, & you end up with nasty tasting beer, with a fruity taste.