Flash :: Optimal Referential 2d Array - Determine What Tile Is At A Location?

So, I'm creating a fairly basic overhead 2d game where users can "draw" a map. (Actually, they dont draw it, the manually input a list of x/y's, but the design aspect isnt important just now.) When a new tile is added, that tile goes into an array of all tiles ingame. The centrepoint is 0,0. Tiles can be added in all directions, so may be at 1,1 or 100,100 or -50,-50. Sometimes I want to determine what tile is at a location. One (imho bad) way of doing this would be to get the x/y and loop through all tiles and check if they are that location.

The way I'm currently doing it is to have a seperate 2d array of null elements, and when a tile is added, its set at that array. (ie tilemap[10][10] = tile[100]) Of course, because the values can go negative, tilemap [0][0] is actually the -1000/-1000 tile. (chosen as an arbitrary limit) Is there a better way of doing this? I feel like using a massive array of mostly empty objects could be more optimal.