Domain marketplaces like DropCatch and ExpiredDomains.net typically obtain their lists of expiring or to-be-deleted domains from the respective registry of each top-level domain (TLD).
Each registry maintains a list of domains that are set to expire or be deleted. These lists are often made available to registrars and other third-party services that specialize in acquiring expired domains. The marketplace may also use tools and software that monitor the domain registration database to identify domains that are approaching expiration.
In addition to obtaining these lists directly from the registry, marketplaces may also use data from registrars or other sources to identify expiring domains. However, the registry is typically the most authoritative source of information on expiring domains.