From Glasgow to Belfast

We plan to visit the UK, Scotland and Ireland in July and August. We'll stay in Glasgow from 2nd August to 8th. Later we want to go to Belfast. Looks it is better for us, going by coach/ferry/coach. I would like to know if I need to buy the tickets in advance, on line. Any possiblity running out of tickets?

I expect you could go to the bus station and buy them on the day but why take the risk of the tickets 'running out'?

http://www.citylink.co.uk/journeyplanner.php