Using unfinished furniture is a great way to get well-made items without paying for a finish that you may or may not like. Plus you can customize the piece with stain, paint, or additional elements. A few places I have tried:

Pop's Unfinished Furniture
This online option has tons of selection, as well as good photographs so you know what you are getting. They have shaker, country, and a few more modern furniture styles to choose from.

IKEA
Several of IKEA's module storage systems (like IVAR) come in unfinished, untreated pine. Finishing them with stain or tung oil can give a great modern feel to a space. Check out how to stain IKEA furniture.

Target
Believe it or not, Target has a small selection of unfinished furniture online. Fairly modern in styling, too!

American Unfinished Furniture
They only have two physical stores, but a large inventory available online.