Not that I do this... all the time... or am overly anal about paint care etc... but...
I have nurmerous places that in order to avoid swirls in your paint (Jet black) do NOT dry your car with a towel... use a leaf blower or something else. my local car wash has one of those reverse vaccums in the wash bays that you can manually air dry your car - it sucks and is a total PITA but does prevent swirl marks.
but back to your question, ebay has paint matched "pens" with the color of one end and a "clear coat" on the other end ... think of it like a double highlighter.. I got one for my old nissan before I got my BMW and worked great.
No scratches on the BMW yet to warrant using touch up paint, but if I had to, ebay would prob be the route I would go. You can find quality paint matches, no worries there.
BMW dealer also has them for sale.
A quick search on ebay, I found this: http://www.ebay.com/itm/BMW-Touch-Up-Paint-Jet-Black-Schwartz-II-CC-668-/390473854645?pt=Motors_Car_Truck_Parts_Accessories &hash=item5aea0e2eb5&vxp=mtr