Getting the first unused Id from a table?

Question

In our database we have a table that lacks an identity column. There is an Id column, but it is manually populated when a record is inputted. Any item with an Id over 90,000 is reserved and is populated globally across all customer databases.

I'm building a tool to handle bulk insertions into this table using Entity Framework. I need to figure out what the most efficient method of finding the first available Id is (under 90,000) on the fly without iterating over every single row. It is highly likely that in many of the databases, someone has simply selected a random number that wasn't taken and used it to insert the row.

What is my best recourse?

---

Edit

After seeing some of the solutions listed, I attempted to replicate the SQL logic in Linq. I doubt it's perfect, but it seems incredibly fast and efficient.

var availableIds = Enumerable.Range(1, 89999)
                       .Except(db.Table.Where(n => n.Id <= 89999)
                           .Select(n => n.TagAssociationTypeID))
                       .ToList();

Answer 1

Have you considered something like:

SELECT
    min(RN) AS FirstAvailableID
FROM (
    SELECT
        row_number() OVER (ORDER BY Id) AS RN,
        Id
    FROM
        YourTable
    ) x
WHERE
    RN <> Id

Answer 2

To answer your implied question of how do you get a list of available numbers to use: Easy, make a list of all possible numbers then delete the ones that are in use.

--This builds a list of numbers from 1 to 89999
SELECT TOP (89999) n = CONVERT(INT, ROW_NUMBER() OVER (ORDER BY s1.[object_id]))
INTO #AvialableNumbers
FROM sys.all_objects AS s1 CROSS JOIN sys.all_objects AS s2
OPTION (MAXDOP 1);

CREATE UNIQUE CLUSTERED INDEX n ON #AvialableNumbers(n)

--Start a seralizeable transaction so we can be sure no one uses a number
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
begin transaction

--Remove numbers that are in use.
delete #AvialableNumbers where n in (select Id from YourTable)

/*
Do your insert here using numbers from #AvialableNumbers
*/    

commit transaction

---

Here is how you would do it via Entity framework

using(var context = new YourContext(connectionString))
using(var transaction = context.Database.BeginTransaction(IsolationLevel.Serializable))
{
    var query = @"
SELECT TOP (89999) n = CONVERT(INT, ROW_NUMBER() OVER (ORDER BY s1.[object_id]))
INTO #AvialableNumbers
FROM sys.all_objects AS s1 CROSS JOIN sys.all_objects AS s2
OPTION (MAXDOP 1);

CREATE UNIQUE CLUSTERED INDEX n ON #AvialableNumbers(n)

--Remove numbers that are in use.
delete #AvialableNumbers where n in (select Id from YourTable)

--Select the numbers out to the result set.
select n from #AvialableNumbers order by n

drop table #AvialableNumbers
";
    List<int> availableIDs = context.Database.SqlQuery<int>(query).ToList();

    /*
       Use the list of IDs here
    */

    context.SaveChanges();
    transaction.Commit();
}