I just got the "official" BB email about this a couple days ago so it looks like they're kicking this plan into full gear now...or sometime soon. What I'd recommend is that you call your local BB first and see if they are participating. I called 2-3 BB's around me and some were participating, some didn't even know what I was talking about. Choose your BB wisely.
I went two days ago to my local Best Buy and they had plenty of them. I purchased my iPhone 4 there, but they did no lookup or anything to see if that was the case. I simply walked up to the Mobile counter and asked if they had the Invisible Shield Fix for the iPhone 4. The salesman got it out of the cabinet behind him and rang me up right there with only asking if I wanted it installed or not. I told him that I would rather do it myself, and he said that was fine. He handed it to me with the receipt and instruction sheet and I was on my way.
You should have luck getting one at your local Best Buy, but I would advise calling ahead to make sure. Mine had an entire box full of them left when I went.
Also, from my experiences: The new "fix" is not nearly as nice when put on as other shield models. The dry install makes it tough to not get stuff stuck under the shield. As well, this does not fix my antenna issue at all. I can still force my phone to enter "Searching..." through holding it with the death grip. Back to the bumper it is.