don't forget the maui onions
you can buy seed from many companies (and someone on gardenweb will happily trade anyone from the uk with chiltern's, for example, hehe)... the spiel is that of course, it is the super-sweet soil (less sulphur? trying to remember) that makes the super-sweet onion, but i'm thinking mostly genetics (& if not, make a raised bed w/ suitable soil & have a go).
btw, in the meantime, i think shallots are a fine sub for 'em, not in the sense of replacing w/ an equivalent substance, but that the flavor of a fine, juicy, thinly sliced big shallot would not ruin say, a summer tomato & homemade mayo on fresh white bread sandwich (the only 'irreplaceable' place to stick a vidalia i can imagine- the classic). anywhere else, why not?
btw, see what chan has to say in the garden section- she'll know her onions
edit: oh crap, someone ask chan- are they short-day or long day? onions are funny about stuff like that, & i forget which all the sweet ones are. oh heck, you may just have to find a pricey greengrocer w/ exotics.