ItemRegistry.cs

using UnityEngine;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

public class ItemRegistry : MonoBehaviour
{
    static ItemRegistry _instance;

    Dictionary<string, Item> _items = new Dictionary<string, Item>();
    
    void Awake ()
    {
        _instance = this;
        _items = Resources.LoadAll<Item>("").ToDictionary(i => i.name);
    }
    
    public static T GetItem<T> (string id) where T : Item
    {
        T item = null;
        
        if (_instance._items.ContainsKey(id))
            item = _instance._items[id] as T;
        
        return item;
    }
}