We are designing an architecture of a high availability distibuted system and we are considering server side vs client side service discovery. There is one problem, I need to solve - how is it possible to discover the Discovery Service (Eureka/Zookeeper/NGINX...)? Is there any way, how not to hardcode/hardconfigure the Discovery Service address?
You've got options.
Bare Metal/Standard Virtual Machines
If you are on bare metal (getting rarer these days, but still common enough in some industries), you can always have an instance of your discovery service on each machine. Since just about all of them handle clustering, you just need to look at localhost for your discovery service.
If docker is handling your "dns" for you, then simply have a docker instance for "discovery".
You can always inject environment variables to tell your services how to get to the discovery service. Both containers and virtual machines let you do this easily.
For non-trivial microservice deployments, it's really common to have a config server. That config server resolves all the parameters so that you can find the discovery service, among other things. NOTE: in this case you would be injecting the URL for the config server so the problem just moved.
Look at your deployment
There are several opportunities to handle the discovery service problem. There very well may be options I haven't listed here.