Electronic but unmounted? (I'm just guessing since this is in the DIY forum.)
Only electronic ballasts I'm aware of for 250 or 400 are really just the aquaria brands (PFO, Icecap, Blueline {not sure if they even exist anymore}, probably a bunch of others), so basically you're looking at an aquaria supply store for these not so much an electrical wholesaler.
A couple months back I saw an ad for an electronic ballast in my Aquarium Fish Magazine. It looked kind of keen since they were claiming multiple wattages and dimmable. I'll try to dig it up and if I find it I'll post back the info. But I wouldn't expect it to be very cheap when faced with those kinds of features. Seems to me in general electronic ballasts are bought at a premium. ..
