Unfortunately most conventional stores don't stock any sizes large enough. The three sizes on the rack is what you get, and if you move to a country with an average size smaller than you (or like something in a shop that treats male fashion consumers as rake-thin models just like the females), you're basically out of luck nearly 100% of the time.

I generally don't find anything larger than 36" waist where I am and that's already got room to spare at the actual waist. Of course leg length is also an issue - anything bigger than 34/36 and you must have 2m legs, right? And then adjustment prices are high too :S