Trend certainly used to offer a long shank 1/4" cutter (1/4" shank though) - I have one.
I don't think the depth needs to match the length of the carbide inserts though - if you think about it, when slotting, most of the cutting is done by the end of the cutter anyway (or very close to the end). So as long as the cutter isn't narrower than the shank, and the shank is long enough, and you are clearing waste from the hole* you're good.
That said, spiral upcut and downcut bits seem to be longer, albeit at a price. Have a look at Wealden's site - if anybody sells what you want, it'll be them.
But personally, I'd use a 1/4" collet: a 1/2" shank is a nuisance, as you can't fidget the bit lower if you need to, the shank gets in the way of good sightlines (or can do), and if you try to go too deep you will scuff the shank on the top of the workpiece. Not worth it, IMHO.
E.
*Obviously in a router table this should be less important as gravity is working for you, but the nearer you get to a zero-clearance insert in the plate, the less space there is for the chips to escape, and thus slots do clog. Steve Maskery did a neat mod to his router table to pretty much eliminate clogging when slotting. He wrote it up here a while back: use 'search' to find it.
