If you are in the grid''s mousedown event, I think you can use code like this to get the cell rectangle.
int row, col;
Point pt = new Point(e.X, e.Y);
if(this.gridDataBoundGrid1.PointToRowCol(pt, out row, out col, -1))
{
Rectangle cellRect = this.gridDataBoundGrid1.RangeInfoToRectangle(GridRangeInfo.Cell(row, col));
}