Hi JRB,
I'll toss in my two cents worth here!
The Onsies and Concealers you've mentioned seeing are generally made of the same material as underwear, a soft cotton or cotton poly blend that is comfortable, just like a cotton tee shirt, and does a great job of concealing your diapers as Paul Martin said.
In my experience the quality has varied considerably and so has the fit....from pretty good to bad. Some became auto polishing rags early on.
Support for diapers is a whole different matter. While the onsies and concealers provide minimal support....and I stress "minimal" there is much value in a garment that actually supports a diaper, since your comfort is enhanced with wearing a diaper that is not sagging between your legs and most disposable diapers will benefit from the compression afforded by support and keep you free of leaks longer than if not supported.
Now we are talking about a stretch garment, a body suit. Body suits have Lycra or Spandex as part of the fabric and so are stretchy and fit closely. A disadvantage is the snaps are in the very bottom of the crotch and can be hard for some to reach. The crotch snaps in the onsie style underwear are generally towards the front which facilitates snapping without standing on your head.
What to try?
If you interested in just concealment, then a onsie is the way to go. I can't make a recommedation as mine were custom made by a lady now retired from sewing. The Concealer, offered by AngleFluff I believe, is a reputable company but I've no experience with their one piece underwear.
If you are interested in support for your diapers that also helps concealment in that the elastic fabric compresses and holds the diaper in snugly against your body, then the body suit is the way to go. My recommendation is to start with Abena....yes, the diaper manufacturer. You can get an Abena "Bodystocking" from
www.xpmedical.com This garment is a sleeveless shirt with straps over the sholders. A nice plus is the crotch snaps are located forward for easier snapping.
If you find you like the body suit approach for support and do not object to the snug feel of the overall compression, then look into
www.bodysuit.com where you can find various styles and colors for sleeves and turtle necks and so on. However these garments all have the crotch snaps in the very bottom, can be hard to reach and snap.
Hope this helps you!
JoeK