I have googled and spent what time I have been online today researching this item. I cannot find it although when I google it I find tons of pictures and videos of them.
I think this would solve a lot of my issues with where to put one.
I need you to send me a link if you know of a company that makes/sells these. The links I have found are not working or they actually go to companies that the phone number is now something else....fun fun, I always find something and it's too old to still be on the market.
I am hoping to find something like this in my area so that I might not have to find one hundreds of miles away that would be of no use. BUT they might be able to let me know where they could be made locally.
If you know of any concrete workers that could do this in the Oklahoma/Kansas/Texas region that would work too.
Got a couple of them finally.
Here's the one I might go with.
http://www.missouristormshelters.com/shelters-step-deck.aspx
I know that a lot of people have started putting storm shelters under the garage. That's actually standard protocol for new homes in Oklahoma, I think. So if there's an out of the way spot in your garage where you could have a "trap door," you might consider that as another option. It might cost more, but it would probably be safer, too.
ETA: This looks like a good place to start looking. http://www.groundzeroshelters.com/
Did you try looking for 'safe rooms'? Put in saferoom shelters in the search and you'll get tons of links.
I suggest that you talk with building contractors. You may find one that has done this for other people or who would know of someone who has.