Central Database class
I would like to point some opportunities here. As you are refactoring your old code, I suggest:


try to avoid the use of literal string inside methods. Instead, use const variables or Resources (I do prefer Resources);
create your variables where you need them. Move your using to the nearest point of interest;
try to use string.Equals, it will be faster; 
get your DbCommand directly from your connection, you have a method to do this;
try to use a Factory Pattern in this class, so in the future, if you need to change your database, you won't have to search for all your SqlConnection objects, you'll just need to change your Factory creator;
don't forget your XML comments


I made an example just to show those points. Sorry for the simplicity, but it's more difficult without knowing all your solution, but here is the idea:

using System;
using System.Configuration;
using System.Data;
using System.Data.Common;

namespace ClassLibrary1
{
    public class Class1
    {
        public const string shopStr = "shop";
        public const string shopDbConn = "Shop.DbConnection";
        public const string factoryDb = "System.Data.SqlClient";

        private static DbProviderFactory factory = DbProviderFactories.GetFactory(factoryDb);

        /// &lt;summary&gt;
        /// Get data from the database.
        /// &lt;/summary&gt;
        /// &lt;param name="sql"&gt;SQL command to retrieve data.&lt;/param&gt;
        /// &lt;param name="connString"&gt;Connection string for the database.&lt;/param&gt;
        /// &lt;returns&gt;&lt;c&gt;IDataReader&lt;/c&gt; with the first result set, or &lt;c&gt;null&lt;/c&gt; if no result was found.&lt;/returns&gt;
        public static IDataReader GetRS(string sql, string connString)
        {
            IDataReader result = null;

            using (DbConnection conn = factory.CreateConnection())
            {
                if (string.Equals(connString, shopStr, StringComparison.OrdinalIgnoreCase))
                    conn.ConnectionString = ConfigurationManager.ConnectionStrings[shopDbConn].ConnectionString;
                else
                    conn.ConnectionString = connString;

                using (DbCommand cmd = conn.CreateCommand())
                {
                    cmd.CommandText = sql;

                    using (DbDataAdapter adapter = factory.CreateDataAdapter())
                    {
                        DataTable tbl = new DataTable(); // don't dispose it
                        adapter.Fill(tbl);
                        if (tbl.Rows.Count &gt; 0) result = tbl.CreateDataReader();
                    }
                }
            }

            return result;
        }
    }
}