Libya is tricky because some US-run booking engines won't take bookings. Travelocity is OK, I think - look at the Libya thread.

Others are usually bookable on ba.com or by phone. Note that unless you ticked the 'check available seats' box, you may not find you can get the seat on the days you want. For Bangkok in November, for example, only about 1 day a week had I-class Club availability, and we are having to fly Wed-Wed instead of Sat-Sat.