with t as (
	select 1 as a, 'a' as b, null as c 
	union all
	select 2 as a, 'b' as b, null as c 
	union all
	select 3 as a, 'c' as b, null as c 
	union all
	select 4 as a, 'a' as b, 100 as c 
	union all
	select 5 as a, 'b' as b, 200 as c 
)
select t.a, t.b, isnull(t.c, t3.c)
from t
left join ( 
		select *
		from (
			select b, c, ROW_NUMBER() over(partition by b order by a) r
			from t
			where c is not null
		) t2
		where r=1
) t3 on t3.b=t.b

위 소스처럼
셀프 조인을 이용하시면 됩니다.
row_number를 쓴것은 b컬럼에 'a' 값이 여러개 들어가는 것을 가정해
c컬럼에 not null인 것중 a컬럼 값이 가장 작은 것으로 채우기 위해서
쿼리가 좀 복잡해 졌네요.

SELECT a
     , b
     , ISNULL(c, MIN(c) OVER(PARTITION BY b)) c
  FROM t
 ORDER BY a
;