Did they find a new one, retire, become monks, merchants, follow their masters to death etc. And why? Was there any major deciding facts, or was the choice even theirs? Did the lords have some kind of agreement with them, that if they die, they'll commit seppuku to follow? Really interested if there was any rules or agreements between the two regarding this situation.
I'm a bit confused by your question. By definition a rōnin does not have a master. If he did he wouldn't be a rōnin.
Ostensibly, it would have been considered 'appropriate' for a samurai who had become masterless to kill himself by seppuku, at least according to the ideological documents surrounding samurai behavior, but in practice many samurai chose to continue to live after their master's death or even to voluntarily leave their masters in whose service they were dissatisfied.
The general disarray and decentralized power structure of the Sengoku period meant there was no clear and rigid system for dealing with masterless samurai, and unlike the Edo period, the bond between the lord and the samurai was much more loose, or at least more variable from domain to domain. While some Daimyo may have shunned the masterless samurai as untrustworthy, others may have readily accepted their service to increase their military forces.