There are 4 ways to get a runner: buying directly from the market, buying from another player, rescuing him during the adventures or by breed.
Kill Mimic: during your adventures, you can find a mimic, always next to him there will be a chest, inside it will have a locked runner, you will receive this chest as a reward after defeating the mimic. You can obtained the key to open the chest randomly during your adventure after defeating some monster or buying it directly in the market.
Market: in the market the player can buy runners, there is no limit initially, so you can get only 1 runner or a pack with 5 runners.
Marketplace: You can buy runners from other players, doing that you can filter and choose which runner you want, the price will be determinated by the player that is selling his runner, in this way, be aware of all the information of the runner you are going to buy.
Breed: with 2 adult runners, the player can breed and generate a new runner.