In the window covering industry, there is a term used to denote blinds that are sold in large hardware stores—they are called "cut-downs". Cut-downs are bought by the boatload as blanks and are kept in stock. The purchaser of these blanks is required to measure their own windows so that the blanks can be cut to the desired width. These cut blanks will then be ready for self-installation, after which there needs to be an additional cut to achieve the desired length.
Professional speaking, I have yet to see a quality product available from cut-downs purchased at the large chain stores. However, a few online blind companies do exist, which are now selling cut-downs that are of excellent quality.
The downside, however, is that these cut-downs (quality or not) still require the home purchaser to meticulously measure, cut and install the cut-downs themselves. Should the measurement be off, or should they become damaged in any way during the installation, the purchaser is 100% responsible. The cost savings that might have been realized from a do-it-yourself project can quickly evaporate with even small mistakes.