I get mine from
www.lbshorticulture.co.uk . You have to set up a trade account with them to get the good deals. But there are lots of other places which sell them, in various forms and combinations. I used to keep mine in the barn, with the plants standing on a gently heated sand tray, but they didn't really get enough light so were still drawn. Now my main propagator is on the bedroom windowsill, which is south facing, so the plants have sunlight in the day and the gro-lites in the evening. It's not a perfect solution and they still end up a bit tall and pale, but soon green up and become sturdy when I plant them into the tunnel in May/June. Some years tomatoes and cucumbers have fruit on them and are 2 or 3 feet tall before they go out, if I've sown them too early. If the weather is mild then I stand them outside or in a cold frame during the day and bring them back in at night.
My Gro-lites are set up with a pulley system (I knew my orthopaedic traction knowledge would come in useful one day
) so the light can be kept low to the plants when they are small, and taken higher as they grow.
The cheapest option is to buy just the bulb and make your own reflector.
I would try to keep my tomatoes at about 15-18C, although once they go to the tunnel night temps will be lower than that and day temps much higher.