A couple of days ago I was on the hunt for a long-sleeved white shirtdress, one that was smart enough to wear to work, but could also be dressed down with a pair of jeans. I ended up finding one in H&M for £19.99. The sleeves are 3/4 length, so it's not perfect, but you still need to wear a long sleeve cotton top underneath anyway because it's see-through. I couldn't find the exact picture, but it's close to the ones shown, has a removable tie-belt at the waist and comes just above the knee.
To dress this down, I'd wear a coloured or white long sleeve top underneath, and then co-ordinating hijab and accessories. It's also great to wear with those 'difficult' hijabs which don't quite seem to go with any other outfit: