Alan Thompson has crafted a great routine for finding a station and offset on an alignment in AutoCAD. This routine will work with any object, such as a polyline or spline, and it also works with Civil 3D alignments.
Load the LISP file and type in the command SF. If you select a polyline, line, or spline, it will ask you for a starting station number; stations increase in the direction the entity was created. If you select a Civil 3D alignment, the stations previously assigned to the alignment are displayed. Next enter a station value and an offset value, and a line will be drawn perpendicular from the alignment to the found point. If you enter a value outside the limits of the alignment, the program alerts you and lets you try again. Well done, Alan.