Just used a few buses on our trip in Macedonia. Whilst there are a few posts I thought it might be useful to provide an update:
Ohrid to Bitola cost 220 denar.
Bitola to Skopje cost 450 denar.
Around 75 denar to a £ at the time of writing.
There is a timetable published in the Brandt guide. In practice there are a number of other buses available, so do not stick to the timetable that is published in that book. None of the buses were even close to being half full so I wouldnt worry about pre booking. Each booking office spoke sufficient English and buying tickets was really straight forward. Finally, for some reason all of the bus stations are a fair walk from the town centre, usually a 20 min walk. We didnt take a taxi but if you dont fancy the walk then there are plenty available!
I dont have a timetable and am aware of one published on a Macedonian website (which i found largely unintelligible), but I do have a list of times between Bitola and Skopje and vice versa if anyone is interested.