@Wouter makes a good point about services being stateless. However, you can use the DI factory capability to accomplish your goal:
specific_entity:
class: SomeEntity # Not used but you still need something here
factory_service: specific_entity_repository
factory_method: find
arguments:
- 42
This will call the find method on your repository with an argument of 42. The returned entity will then be injected. I'm assuming you already know how to setup a repository as a service.