Does anyone know of a better fieldtype to handle long lists in a dropdown than the default dropdown list fieldtype?

I need to 


work within Grid
field must be searchable in front end (via ee or low search)
be able to add items within the list at times
handle thousands of items in the list (maybe 5,000 - 20,000)
hopefully offer an improved way of choosing the item


(I considered using a second channel and a Relationship in the Grid, but the problem is searching isn't a possibility for Relationships in a Grid. That would be ideal in that I can change/add to the items in the second channel, but the search issue is a deal breaker.)


Question: MSSQL Select with "vertical"-where
Answer: SELECT PRODUCTID
FROM T
WHERE TAGID IN (59,61)
GROUP BY PRODUCTID
HAVING COUNT(DISTINCT TAGID) = 2


Or

SELECT PRODUCTID
FROM T
WHERE TAGID = 59
INTERSECT
SELECT PRODUCTID
FROM T
WHERE TAGID = 61