Where can I find data sheets for a Sanyo Denki 103H5208-1241 Stepper Motor
If you have a specific part number and don't get a good result with the full thing, start stripping off parts and see if that helps.  For instance, ATMEGA169PA-AU defines an exact part you can order, but searching for "ATMEGA169PA" or "ATMEGA169" probably would give more helpful results.

For your case, "103H5208-1241" seems to provide good results when searched, but "103H5208" does as well, many of them seeming to be whole catalogs or large sections thereof.

The reason for my example is because most of the additional letters tacked on simply indicate mechanical features or different grades, but the core part is still the same.  Your stepper motor might also differ in some mechanical aspects (threading, mounting, whatever), but even a different part with a similar part number should have closely related features, so you could glean something from that.