Enum.
    GetValues(typeof(CurrencyId)).
    Cast<CurrencyId>().
    Where(c => c != CurrencyId.UNSUPPORTED).
    Select(c => new
    {
        CurrencyId = c,
        SortId = c.
            GetType().
            GetField(c.ToString()).
            GetCustomAttributes(
                typeof(CurrencySortAttribute), false).
            Cast<CurrencySortAttribute>().
            Take(1).
            Select(a => a.SortId).
            DefaultIfEmpty(int.MaxValue).
            Single()
    }).
    OrderBy(a => a.SortId).
    Select(a => a.CurrencyId).
    ToArray();

public enum CurrencyId
{
    [CurrencySortAttribute(1)]
    USD = 840,

    [CurrencySortAttribute(2)]
    RUR = 643,

    [CurrencySortAttribute(3)]
    UAH = 980,

    [CurrencySortAttribute(4)]
    EUR = 978,

    [CurrencySortAttribute(5)]
    KZT = 398,

    UNSUPPORTED = 0
}

[AttributeUsage(AttributeTargets.Field, AllowMultiple = false)]
public class CurrencySortAttribute : Attribute
{
    public CurrencySortAttribute(int sortId)
    {
        this.SortId = sortId;
    }

    public int SortId { get; set; }
}