Here's a bunch of excellent images, plus a bit of criticism so you know how each one could be even better. (Almost) All of them are images I used to make my own imports, so they're all "good enough."
Link to all these images, full-size: http://imgur.com/a/dLvLj
Or right click+open in new tab to see full size.
OFFICIAL ART
Official art is always a good thing to check out. If an artist goes off of official art, it's very hard for them to "mess anything up" as far as accuracy of portrayal.
This is a pretty good picture to be able to reference. The only problems are that when you zoom in to her head, it is somewhat small, and that the style is pretty non-SDT. However, it's definitely a good image.
ARTBOOKS
Related to official art, artbooks often contain a wealth of information about exact how the subject's head looks, and have such wonderful shots as these:
Especially of note is how this clarifies the difference between the left side and the right side of her head; for many characters with wacky anime hairstyles, this matters.
SCREENSHOTS
This only works for anime/cartoons, or anime-style cutscenes from a game, but it can be a perfectly good way to get a profile view and is also under the umbrella of being "official."
This is almost perfect. The only things to watch out for are the fact that it's the wrong side of her head (doesn't matter since her hair is symmetrical) and the colors are faded (supply an image with brighter colors).
Another screenshot from the same anime, a great image for the girl on the left. Colors are brighter, it's the correct side of her face. The only things to look out for are that even though it's an official image, for some reason it obscures green headband, which is an important aspect of her appearance. Also, the hair coming down in front of her body is resting on her shoulders in a way that would not work with SDT; this is something that could be left to the artist to fix, but it's good to know.
(ignore the stupid text added, I didn't do it) A great image. The only caveats are that since her head is tilted downward, the artist will have to adjust for gravity messing with where everything lies.
BREAST CHARTS
These are wonderful in that they almost always show a great multitude of characters and their hairstyle from the side. Sometimes they can be a little too deformed and stylized, but they are usually excellent. It's also the first category we've run into that is not drawn by an "official artist" (99% of the time), so be wary of whether or not the portrayal is 100% accurate.
This is just a little bit stylized and it's the wrong sides of all of their faces, but otherwise, it's a veritable goldmine.
FANART
Breast charts are usually fanart, but this is the last-most-preferable category. Most hentai is fanart, and everything on deviantART is obviously fanart. It's also pretty likely that this will be the bulk of what you find. Just be mindful that in this category, scrutiny for quality is key. There will be a lot to wade through that, even though they show the profile, are in the wrong style, are not accurate, etc.
This is pretty good. Her hair, however, is moving, so an artist would have to manually adjust to make her hair look stationary. Also, the back of the hair is cut off and an artist would need to consult another reference or freehand that part of her hair.
Pretty excellent. The only problem is that the style doesn't match SDT's completely, and the proportions of sizes are a bit wacked (the bangs are way too huge compared to the "official" portrayal).
This is good hentai (in more ways than one). It realistically and accurately portray's the subject's hair, except her braids, which look strange since they're in contact with the ground and not hanging. There are good hentai artists to rely on who always draw characters accurately and in a somewhat "realistic" way, such as Azasuke, Jyusuke, etc.
This is actually just "okay." I ended up relying on it for one of my first imports and that import needs to be revised desperately. It's in oekaki, so the lines are very thin and hard to trace over accurately, it's very rounded and stylized in a way that doesn't fit SDT, the proportions are off, and it's the wrong side of her face. Feel free to give an artist this picture, but don't expect them to be able to use just this.