I change hand positions a lot, especially while riding casually,
and I would like to get some of grips on the top of the Nitto
123s. Do they actually make these, or are they just regular
grips with the ends cut off and slid to the top of the bars? I
ask because I've never seen them for sale anywhere.

no they do not make special grips for the tops.
the ends are just cut off ...or they are grips
which have no ends.

lets of grips do not have ends, but you can make super clean cuts on closed grips so that they may as well have been open.
1. remove bar plug.
2. stick grip on (water).
3. tap the end of the grip with a small hammer co that the bar cuts a neat little hole.
4. remove grip, dry it and then slide it up the bars with high 91% isoporyl(rubbing alcohol), enjoy the ether.
5. set in place and let dry.
6. reinsert bar plugs.

i cut the ends of some champs. works great.

Thanks for the tips, guys. I'm going to chop some soon.

this is what tape is for

grips > tape.