public static class IntExtensions
{
    public static string GetOrdinalSuffix(this int @this)
    {
        switch (@this % 100)
        {
            case 11:
            case 12:
            case 13:
                return "th";
            default:
                switch (@this % 10)
                {
                    case 1:
                        return "st";
                    case 2:
                        return "nd";
                    case 3:
                        return "rd";
                    default:
                        return "th";
                }
        }
    }
}

The switches are a little ugly, but they are quick.

Ordinal suffix of an integer

C#