create table #numbers (n int primary key)

insert #numbers (n) 
select 1 union all select 2 union all
select 3 union all select 4 union all
select 5 union all select 6 union all
select 7 union all select 8
--... to as large as required

create table #t (id int primary key)
insert #t(id)
select 1 union all
select 2 union all
select 3 union all
select 5  

--the next id
select MIN(n) AS id
from #numbers n
left join #t t on t.id = n.n
where t.id is null


--clean up example
drop table #numbers, #t