Coffee contains a lot of sugars. The darker you roast it, the more of it caramelizes, turning it first into bitter-sweet, then just bitter. So, if you like dark roasts, you will rarely taste sweet coffees. Moreover, the riper the coffee, the sweeter it is and the more expensive it is (ripe coffees have to be handpicked, since coffee cherries ripen unevenly, randomly over the course of three weeks or so).

In general, roasters avoid roasting fully ripened and expensive coffees very dark; so you are getting unsweet roasts of unsweet coffees. The problem is that lighter roasts are not only sweeter, but also more acidic, which tends to muffle the subjective perception of sweetness. The way around this is to have slightly longer light roasts. My sense is that Dragonfly, Klatch and Gaslight roast in this style. There are others, but they stand out in my mind. If you want to taste sweet, I'd start with their light to medium roast offerings, perhaps choosing a low acid coffee like a Brazil origin, or Bourbon varietal to boot.