I've both ordered blinds online as well as having them cut to fit at Home Depot/Lowes. For inside mount just make sure you measure at the top, middle, and bottom and go with your shortest width. Be sure to measure at the depth you plan on installing the blinds as well. Agree with piffle that they are easy to install.
Just my opinion here : I'm familiar with the type of blinds you describe, but have not priced them out. $1K for 4 blinds seems really steep to me, but again I usually get the typical 2" slat wood or faux wood blinds. If you are set on the style you describe just make sure you shop around if you haven't already. I recently purchased some room darkening roller shades and would not have thought to look at JC Penney if my wife hadn't mentioned them. After looking at several sites that's who I ended up going with.
Hope this helps.