Thanks everyone for your views on this. I'll try the local timber yard first - good idea - but I doubt if they'll have what I want as it is for a tiny window and must be smaller than standard. To be more precise, it's for one of those foot-high openings in the loft of a 300-year-old farmhouse in the mountains of Spain, where you don't want to let too much heat in through a 3ft-thick wall.
The fun starts, of course, when you try and make a decent job of jointing the corners of a frame made out of a moulding incorporating a rebate. I've tried this before and eventually got it right, after many attempts. It would be so much easier to do mitred joints, but they would have no strength at all. Is there a simple way of calculating the shape of, say, a half-lap joint, which I've found to be quite adequate on such a small size?