My K5 (Dry Mill) came through Ivoclar bundled with their PrograMill suction unit. It's expensive, but built like a tank. So far I really like it, it's been just over a year of use.
Otherwise, you would be just fine with a Quatro iVac. You would just need to let them know what mill you're using, so they can provide the right cord for the auto start, auto stop of the vacuum to mill.