Neil:
I'm with you. I've been hoping for a big-scale version of the old Greyhound Scenicruiser for years, but so far, diecast seems to be the only real answer.
A German company called MEK did 1/87 resin versions of both the Scenicruiser and the smaller "fishbowl" buses around 2002, but they seem to have largely disappeared by now. You could try truckstopmodels.com if you want to try the small scale stuff.
Fujimi and Aoshima do a pretty good variety of 1/32 modern buses, but almost exclusively Japanese designs. Hobby Link Japan (to name but one) usually has them in stock. These might have useable components if you're looking to scratchbuild. I've toyed with the notion, but so far haven't gotten that ambitious.
Best of luck
Greg