On many newer doors in kitchens and bathrooms, you can simply pry out the roller with a screwdriver. On older doors, however, you may have to disconnect the stile from the bottom rail, then slide the roller out from the end. On these, the same screw that connects the rail to the stile also holds the roller in place.
Bring the old rollers to a glass repair shop to find remodeling replacements. If the shop doesn't have the model you need in stock, they may be able to order one for you, especially if you can ID the manufacturer. To install the new rollers, simply reverse the sequence you used to remove the old ones. Adjust as needed after the door is reinstalled.