I buy from
GroWell > Hydroponics, Grow Lights, Grow Rooms & Plant Lighting but i go to their shop in fulham and buy everything over the counter.
Just be carefull with flea-bay, lots of fake and substandard unsafe gear is sold on there, make sure the ballast is BS kitemarked and carries the CE stamp.
Remember safety first, especially if the grow is in your home.
Its not worth saving a few quid buying unsafe gear, not saying you cant get quality stuff on e-gay, just be carefull and if a deal seems to good to be true it probably is, just that.
And dont forget to get a relay/contactor if you wanna run a 250w or higher HID in conjunction with a timer.