www.InteriorDecorating.com sells boxed vinyl blinds. The stock size blinds have a size marked on the box but the actual slats and headrail are 1/2 inch narrower. This allows for the blind to install properly and move within the window. This matters when you are installing inside the window frame. These blinds can also be custom cut if you can't find the size you need.
Always remember to measure your window and not the current blind. Order based on the exact measurement.