The simplest way is by asking Chain Rider DeBot. In Surf app developer mode it can once grant any account 100 Rubies. If you need more for some testing tasks, you can contact support or the community at large. Please be informed that due to using unfair practices the distribution of Rubies is not available in Surf web version.
Since devnet is used for testing purposes, it can, on rare occasions, undergo full restarts, that wipe all account and transaction history. In such an event you can request test Rubies from the Chain Rider bot again.