There are several, but none currently available for iOS, at least not through the app store. (I don't know if you care about iOS, but it's the platform I jumped to when Palm ceased to be sustainable.)
The Palm OS emulator for Windows and Linux can probably still be made to work, although in the latter case, good luck getting it to run in the absence of suitably decrepit shared library versions which your distro's package manager probably no longer provides - your best bet might be to find a DVD image of a distribution from 2003 or so, and install that in a VM to support the emulator.
Linux RPMs can be had via Sourceforge [1]. Windows binaries are apparently available from several sources; the one provided by download.cnet.com [2] appears to be free of social diseases and reasonably functional. (It'll complain about a missing skins directory, but there's a generic skin packaged with the application which will work for your purposes.)
You'll also need a ROM image. To my outright astonishment, some of these appear still to be available on the web! [3] "Palm OS 3.5-en-color.rom" boots successfully in the Windows emulator.
And, finally, you'll need the Space Trader application itself. If you don't still have a copy, it remains available for download on the developer's site [4]. Once you have it, extract the version of your choice from the zipfile - the color version works fine with the ROM I mentioned. Then (at least on Windows) right-click the emulator chrome and choose "Install Application/Database", then "Other...", and navigate to the extracted .prc file. The emulator will load the app into the virtual Palm's memory, and then you can just click the icon to run it. It seems to work quite well!
There are also several variably crummy knockoffs of Space Trader available for iOS, none of which does a good job capturing what made the Palm version fun. I keep meaning to reimplement it as a browser app or something, but...
https://en.wikipedia.org/wiki/Space_Trader_(Palm_OS)
On that note, is there a Palm emulator anywhere?