Your best bet is to phone a couple of glaziers (yellow pages). You might find kits in a DIY shop but you'll almost certainly need the glass cut to the correct size - back to the glazier. Easy to fit. Cost is dependent on size and whether you have sash windows or side opening. Probably a quarter to a third the price of new double glazing but not as efficient. Benefits apart from the cost are that you retain the original outside appearance which is often preferable to new white plastic windows. However if your original windows are in poor condition then replacing them will be a better long term solution. Also don't be tempted to buy perspex rather than glass. Over time perspex becomes scratched with cleaning and slightly opaque through the effect of the sun which isn't very attractive.