try dykes. you should be able to build up the correct profile with a combination of different pieces.

another option is to use a router to recreate the profile out of one piece. this may be more time consuming/expensive for just a few feet, but is a great option if need to a lot of molding to match.

The other option which is the most expensive is have the profile blade made custom for cutting that shape. This is usually dont when you need like 200 feet or more. It is good to have if you have neighbors with the same mouldings and they need it too. Then you can both chip in. Otherwise you will have to try and get something close and reshape it to match. You probably will not find the exact moulding. Unless you go to a place like Olde good things or any place like it that sells "reclaimed" lumber. There is a slight chance they may have it.

I second the Dykes Lumber suggestion; they've helped me out in several instances.I believe ( don't quote me on this) that Dykes' primary goal as a business was to duplicate moldings found in local brownstones. My Clinton Hill (or CH, as I like to say) frame house was built in 1874 and Dykes stocks every molding found in the place.

While the Dykes idea is good, it pays to bear in mind that current dimensioning standards are slightly smaller than the standards of yesteryear. In other words, a 3/4" quarter round molding used to be pulled at 3/4", now it's pulled at 11/16". It doesn't seem like much but in order to accurately match existing by doing a build up (making up a large profile by compiling several component profiles), your GC could have a hell of a time with reshaping the currently available sizes (and time = money in this case). If you're interested in an exact match, it will probably be cheaper in the long run to get a profile blade cut and run the stock.

I agree with junker, below - modern "stacked" moldings always look like exactly what they are, due to the difference in dimension standards; and reshaping is a royal pain. If you care what it looks like, go for the profile blade.

I think that building it yourself is a great idea, but I would think you could probably get some existing trim that is very similar from another building... Here in Albany NY I imagine there are a lot more abandonded buildings where scrap comes from, but we have a place that is run by our local historical organization and its called the Architectual Parts Warehouse. They have stockpiles of trim in all shapes and sizes, bannisters, window sahes, moldings, doors bathroom parts, whatever. All of this comes from contractors and homeowners who are working on houses and end up with extra parts that they will not use... they donate this stuff to the historical foundation for a tax writeoff. There has got to be a place like this in Brooklyn. Up here the prices are pretty resonable. This is not to say for spare parts you need to come up to Albany if there are none in Brooklyn, but there are probably a number of communities just upstate that may have authentic trim that is the same or similar style to your own. Hudson NY has become a real mecca for this stuff, but they will probably be overpriced.

Depending on how short you are, you could use decorative elements to make up the short fall. Medallions on the top corners, another medallion in the center, and flat stock at the bottom, for example, could easily make up a foot or two.