Jump Ultimate Stars was a very popular game (overall in Japan) thanks to its big roaster, game mechanics and combos potential.
Maybe it went under the radar from some people because it never got out of Japan... but many people got it from different ways... (It also helped that the DS is retrocompatible).
Currently it is possible to play this again thanks to Wiimmfi (yeah, shocking Nintendo ending official servers right guys?).
Anyway our discord is getting pretty active as of lately, and the server not needing to patch the ROMs (you can play with original hardware) and some emulators like MelonDS being able to interact with original hardware makes it widely easier to play this day.
Here I share some YouTube accounts where you can see how we play this game:
If you want to give it a go be sure to join us in the Discord, or perhaps you are not interested in playing with others, if that is the case you can enjoy the main campaign as many folks find it very good too and talk about it there!
JSS was very neat the combined specials were awesome and sadly they didn't make it to the sequel, but it was a wrong decision to not implement wifi gaming in such a kind of game... that's why JUS thrived regardless of the difficult environment in which it was conceived...
I think it might be a bit difficult to get a physical copy of it... maybe you can resort to emulation for this.