I am sure you will find scenic railroads and museum web sites listed, then click on each one to see what they offer, and you will find out if they have steam locomotives operating. But most if not all have a diesel alternative, and the steam runs only certain times, so check the operating schedules carefully, and your best bet would be to call the office of the railroad to find out if the steam will be running the day of your visit. The worst thing would be to travel really far to a museum or scenic railroad only to find out that the steam loco is not operating that day.
Also another option is to do a search on this web site for "active steam locos" or similar search title.
I hope this helps.