How to return list of implementations?

I'm creating an app which will be used to place orders in various stores. Every store have different way of placing an order. Currently it looks like this:

@Document("order")
public class Order extends AbstractAggregateRoot<Order> {
    private UUID id;
    private Shop shop;
    private Product product;
    // other fields and methods have been skipped
}

public abstract class Shop {
    private String baseUrl;
    abstract void placeOrder(...);
    // other fields and methods have been skipped
}

public class XYZShop extends Shop {
    @Override
    void placeOrder(...) {
        // implementation of method
    }
    // other fields and methods have been skipped
}

When client is creating an order he should be able to select in which store he want to place this order.

How to get list of available shops? Or maybe should I implement such logic in different way?

I very inconvenient way to this would be the following:

public Test3()
{
Class<?> parentClass = this.getClass().getSuperclass();
if(Extenders.extendingClasses.containsKey(parentClass.getCanonicalName()))
{
Extenders.extendingClasses.get(parentClass.getCanonicalName()).add(this.getClass());
}
else
{
List<Class<?>> extenders = new ArrayList<>();
extenders.add(this.getClass());
Extenders.extendingClasses.put(parentClass.getCanonicalName(), extenders);
}
}
public static void main(String[] args)
{
Test3 tmp = new Test3();
for(Entry<String,List<Class<?>>> entry : Extenders.extendingClasses.entrySet())
{
System.out.println("Class "+entry.getKey()+" is extended by: ");
for(Class<?> clazz : entry.getValue())
{
System.out.print(clazz.getCanonicalName()+", ");
}
}
}

with the super class

public abstract class Test2 {
}

and a singleton

public class Extenders {
public static Map<String,List<Class<?>>> extendingClasses = new TreeMap<>();
}

This method however has the problem that you need to create an instance of the extending class first. So it's basically not listing all extenders, only those who get used.

The output in this case would be:

Class Test2 is extended by: 
Test3, 

I might be wrong on this: But as far as I know, the super class doesn't know about it being extended.