I just bought a tri-specific tank and shorts, but I did my first 4 or 5 sprints without. Just swam in my speedo tank suit, then put on bike shorts and jersey over the swimsuit. You don't even need the top - I just liked having the pockets for my gel. This makes for a slower transition, but it doesn't really take that long to throw on bike shorts. It CAN be slow if you try to haul your bike jersey on over your wet self and get your head stuck inside and you can't see anything and you're standing on one shoe-ed foot and one bare foot and your adrenaline-addled brain is soooo . . . confused . . .

Or maybe that's just my transitions.