How low do you want to go? What do you have 8/9/10 speed? I set up a bike for a friend a couple of years ago who was going over to watch the tour and wanted to ride the same climbs as the race. They had a 9 speed record group with Mavic wheels and didn't want to buy a tripple so I put a 11/32 shimano lx cassette on and both the shifting worked fine as well as the short cage managed up to the 32 no probs. If you have a long cage rear der. there should be no probs going up to a 32 on the back you can also go down to a 24 front cog. In 10 speed campag make a 13/29 and in 9 speed they make a13/28 but as you said you can also go for an aftermarket cog set.
good luck