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Free pictures are not easy to find. I bleieve Alliance will generally sell digital files of pictures though. Decadance, Park West, etc, have some amazing pictures, but as far as I know they only sell prints.
Personal use? Just use the internetFlickr, google images, this forum has an images repository. For commercial use, check istockphoto.
Actually, whether for private use or not you should be very careful of securing the appropriate permissions. Whether marked as such or not, the images are not "free for use" unless explicitly marked as such (e.g. as public domain).be careful with copyright and dancer's permission if you will have it posted somewhere.