My gym has a device, it's about 6 foot long and T-shaped. It has a groove to hold the barbell on the top of the T, and you put it underneath where it holds the bar high enough to get 45s on and off. See if you have one at your gym. It's very similar in concept to this but without the wheels:
I don't actually use it though, since it's not that hard to add one or two pairs of 45s to a bar on the floor at my gym (guess it depends on the floor surface).