Chech out Fulham electronic ballasts here -
Fulham - Ballasts and Lamps
In looking at their web pages, I think you want a WH7-120-L, but I'm not sure.
You'll find that this web site only provided information. Since they are a manufacturer, they want to sell ballasts in case lots.
Once you find your ballast, do a web search on it's product number, and you'll find some vendors. Here is an example for the above ballast -
Fulham WH7-120-L Workhorse 7 Electronic Ballast 120V Long Case - ProLighting.com