How much would completely replacing a sliding door cost, on average?

Paint and body shop!... eg.. Maaco..

If you are mechanically inclined and want to tackle this yourself, I would recommend one of the many self service auto wrecking yards such as Ecology Auto Wrecking or the various Pick - A - Part or Pick Your Part locations. I always see a lot of Windstars in these types of yards and you may be even be able to find one the same color if your are lucky! Even if you cannot install it yourself, a local body shop should be willing to install for you for a few hours labor. I have done this with some body parts on older cars that I have owned when it is not worth it to have a body shop do the entire repair. I hope this helps!