Hi mjvr2c.
Welcome to our Mattress Forum! 
You may have seen this already, but just in case you didn’t here are some of my thoughts and comments about air mattresses here. which you already visited. While any mattress can be a good match for a specific person because each person’s needs and preferences or the criteria that are most important to them can be very different … in general terms I would tend to avoid them unless there is a very compelling reason that an airbed would be a better choice in “real life” (outside of the many “marketing stories” that you will hear about them) than the many other options or types of mattresses that are available to you.
Fixing a broken topper on a an existing air bladder is one thing, but as air bladders go this is not something that I usually recommend and again adding a topper to it is a process of trial and error. If you chose to go this way and are committed to an airbed and are convinced that there are no other types of mattresses that will meet their criteria then there are some other airbed options that are listed inpost #3 herethat may be much better quality/value choices than Sleep Number/Select Comfort.
As far as instructions for selecting a topper, there is more information about choosing a topper in post #2 here and the topper guidelines it links to, which along with a conversation with a reliable and knowledgeable supplier (that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market), can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success.
Because of the uncertainty involved with purchasing a topper where you can’t test the combination in person … a good exchange/return policy can also reduce the risk of an online topper purchase so I would also make sure you are comfortable with the options you have available after a purchase to exchange or return the topper and any costs involved just in case a topper you choose also doesn’t turn out as well as you hoped for. There are links to topper suppliers and some with return policies in the link I provided in the previous paragraph.
I deleted your duplicate post from another thread because posting the same question in multiple threads on the forum and it makes it quite difficult to manage, reference input, keep track, and accurately answer your questions.
I hope this helps and I look forward to hear what you decide.
Phoenix
Note to Amity: Thanks for sharing your insights and experience with your system. I am glad that the old Sleep Number with all the additions and adjustments work well for your son and that you gained enough experience and understanding to tweak your new mattress to suit your needs