Roller Bot is one of the three skins released under the Masters of the Galaxy part of the Last Master offer. It could be purchased individually with game coins in late December 2017.
Roller Bot could be purchased in the Skins Shop under the 'Last Master' tab for 690 coins. It was available for a limited time and may not be available for purchase in the future.
Details of this offer can vary depending on location, platform or time.
Note: The Masters of the Galaxy sub-offer was released again for a limited time in mid January 2021. For this release the price of Roller Bot was increased to 1090 coins.