There are no supplements I can think of. Best to buy a good steampunk setting you like and convert.
Conversion is normally far easier thanmost imagine if you do not over think it. The spell Permanent is level 30 and can be achieved via ritual magic, or lowered in level if you wish. So trains with hot water tanks and a permenant BOIL LIQUID spell will work perfectly. Hot air baloons are managable with lots of spells. Do you want fire arms? Go with Fire Arrns Law or Ten Million Ways to Die. FireArmsLaw is best with normal RM armor types, TMWTD is best with RM armor, modern armor and Space Master armor, but the gun tables are more condensed (light pistol, heavy pistol, etc instead of a the progression muzzle velocity FAL offers).
The twice I have ran steam punk setting, I went with the wild west and Rome with trains, hot air ballons, magic schools, etc. It was the ancient world of mythic greece were the gawds had been locked behind the door of time with the titans but the rest of the world remained a magical place with fauna, flora and the ability of gawds to channel to followers here.
I made mostr of the technical skills needed crafts and kept the old world master/apprentice/guilds approach to the learning of the skills. It also explains why scientific advancement is so limited as guilds closely guard their secrets and prevent others from acquiring or advancing them, sometimes accidently, sometimes as an excercise of their secretiveness, sometimes as overt actions.
I admit I didn't really groove to the setting, and my few attempts were quickly aborted for more traditional fantasy/sci-fi blending.
