I don't think that there would be a site dedicated to limited release titles.
If a title is issued as limited (e.i. it states right on the cover that it is number XXXXX out of 30,000), then you should get the movie as soon as you can if it's important that you own it.
In cases such as Criterion, it is often announced that a title, or group of titles at a time will be discontinued (at least that has been the case in my experience). I don't check the criterion website too often, but I suspect that they post an announcement when they plan to stop making a title.
I say just buy the titles that you believe to be limited as soon as you can. I'm usually not one to say this but, it's often not a good idea to wait until the last minute.
