Sarah, I can personally recommend Just Blinds for custom sized roller shades at a reasonable price. We use two of these in our rental and they can be customized anywhere between 8"-191" width.
Another simple and fairly affordable ideas is using opaque window film. Static cling window film makes for a non-permanent solution and are available in a wide variety of sizes; there's a variety of other types and styles of window films that should fit your window and sense of style. And if you're feeling crafty, you could also look into using a preexisting IKEA ANNO SANELA window screen as noted in a previous reader question.