Many in the BDSM world have come up with numerous devices and instruments. One of the more popular is that inflatable butt plug. However I had found as with many butt plugs on the market, that they just aren't anatomically correct, which makes many of them hard to hold in.
The rectal sphincter is about 2 to 3 inches long, and most of the butt plugs only have about a 1 inch narrow part at the base below the taper. The natural reaction is for the muscles to push that out, since part of the widest area had not gone beyond the end of the sphincter muscle.
This had created a market for the butt plug holder/retainer, which can be found through numerous larger dealers of BDSM equipment and supplies. Even vaginal speculums are sold as rectal dilators, something I would personally be a bit hesitant about using as the rectum walls are much more delicate that vaginal walls.
And really, how many men, (since there seems to be more men here than ladies), would actually know which direction these should be inserted? Surely something 'if' used, should probably be used by a partner, as opposed to self-used. Besides, unless someone is double jointed, or is at least very flexible, would be somewhat hard to adjust.
A browse around ebay, and you will find 'real' rectal dilator instruments, those actually made for rectal use. And they are not that expensive, being all imports these days.
A Google search for 'ass spreaders' and/or 'fisting trainers', will take you to very elaborate medical instruments 'advertised' as dilators. Very expensive, and actual, real medical instruments, however really designed for other medical uses, such as retractors.
But people will still use these for rectal dilators. In looking at pictures of some of these, I certainly would not take the chance of using some of these! Some look as though they could be dangerous, others you tend to wonder if they would even work.
I had seen several with blades only 3 inches long, which after adding lubrication, would certainly not stay in place by themselves or without someone holding them there. One such instrument opens up to 5 inches!
Strong and solid, (overseas stainless steel), however with those 3 inch blades, something that would surely go shooting across the room because there's just nothing to hold them in place.
Considering the blades only being 3 inches long, and the sphincter muscles being 2 to 3 inches long, adding in ass cheeks, and you're really not going to get much inside.
Others have scissor type handles, with 'click' locks on them, so you can adjust and lock it into that width. Others have larger, longer blades on them with a curve, which will reach beyond the sphincter muscle. Something that would surely stay inside because of the long curved blades, however you would already have to be dilated to about 3 inches just to insert it!
Again, a retractor, 'not' a dilator!
As mentioned, fingers and dildos work great, however if maintaining common sense, medical instruments made for rectal use can also be found. I had found ebay the best source for these...