Go to your local car paint suppliers - the one i use in my area is called ColourTone.

If they are half good they will have colour cards to match you existing paint shade - obviously paint fades over the years so if you just buy off the shelf paint rather than having it made to match your fresh paint will stick out a mile against your old paint.

ColourTone aerosols are £11 a tin, which is dearer than off the shelf but, for me at least, the extra coin is worth it the superior match and finish.