It's been a couple of years since we went through our installation, but I've done one in each of our last two houses.

First of all - I'm assuming you're reaching out to local companies that install the shelter, and you may have better luck with the shelter manufacturer about how they are listed. We installed these in ground garage shelters (Link), but this was done through a local installer (Link). At the time, these were listed by TTU's wind laboratory (Link), but it appears that they no longer offer this testing according to their website, and defer to UL & Intertek who offer those services. That TTU site still has some good info for standards information.

Secondly - For both of ours, we were able to take advantage of a federally-funded, locally managed rebate program (Link). I don't know what's available in your area, but this is worth some research. $3,000 savings each time. Also, this funding required the shelters to meet the FEMA guidelines (FEMA P-320 & P-361), and this documentation was submitted by the installer to the program, so there's additional incentive to look for a similar program in your area.

Thirdly, we really like the in-ground shelter, as opposed to the surface mounted ones. It doesn't take up any floor space, and the ground offers better storm protection. Plus, it was cool to see how they can install them. It may not work for you, but it's an option to be aware of if you haven't considered it.