Garage door rollers always can be purchased through your local garage door company.

Every garage door tech. carries a bag of rollers on his truck at all times. Replacing a set of rollers is a very common service that technicians provide. If you have 1 or 2 broken rollers and need a quick fix than you can find them in most retail hardware stores. They are sold individually so if you need a whole set of new rollers than you should call a garage door company.

Going to a hardware store and buying 10 individual rollers

And replacing them yourself is probably not a good idea. It may look easy to do and most of the rollers on a garage door are easy to replace. But there is danger in replacing the rollers at the bottom of door. These rollers are attached to the doors bottom bracket which attached to the high tension torsion spring cables that lift the door.

bottom bracket

Any attempt to remove this bottom bracket will absolutely cause some damage especially to you. I have on more than one occasion arrived for a service call and the homeowner had taken a wrench and removed that bottom bracket and caused some damage to themselves and the door.