I dunno, 20+ hour bus journeys are not my idea of fun regardless of the amount of alcohol...
I'd fly there, get the journey part over with, but I hate travelling, I just like being there in the new places.
I usually either book directly on the airline's website (british airways is good for this) or go through a 3rd-party website which searches many other sites for offers. Try
www.ebookers.com (I have bought plane tickets from them, they are reliable),
www.cheapflights.com etc.
The way it works is that the airline allocates some number of seats to each agent selling them, so just because one agent/site says no seats at a particular price on a particular flights, it just means they've sold their allocation, so you still might get some elsewhere.