A motorcycle jack works, as do many other temporary and readily available (free) options.  When making a purchase of this sort, think about how many times it will be needed versus the cost and the space available for storage of it.  
I've rebuilt several bikes with only a floor jack and a piece of wood to hold the wheels off the ground.  If you'll need to be able to move the bike around while the wheels are off, then a motorcycle jack is the way to go.  If not, a standard floor jack and a piece of wood is all you need.