I'm trying to find the kind of inexpensive made-to-order furniture
places they used to have when I was growing up in Boston, but in New
York City and preferably in Manhattan. I have a lot of books and a
tiny apartment, so I need a simple but huge bookshelf as close to 9
feet tall as possible (the height of my ceiling- I'd probably have to
L-clamp it to the wall) and a minimum of 84 inches tall, as well as
being 12" deep and 50 inches wide. Alternatively, two bookshelves with
the combined required width (say 20" and 30" for example) would work.
It would have to be in parts in order to fit it into my apartment.
It'd also have be under $300, and preferably significantly less. If
you can find this bookshelf or if you can direct me to a store that
says it can make it for me under the price limit, you win.

Thanks!

Why don't you go to IKEA or someplace and just get floating shelves
from them...that way you can have whatever length and height you want.
 If they're  not long enough, just butt them end-to-end.  And you
could even make it look more like a "bookshelf" by painting the back
wall and around the sides to give the illusion of a separate piece of
furniture.  Just another option...

Well, I was thinking of getting floating bookshelves, but my friends
can get rowdy sometimes (I'm 22) and most of those floating shelves,
especially inexpenive ones, feel like they're going to break off if
you hit it with a hankerchief.

If appearance is not too important, plastic bottle crates (Evian,
etc.) and boards.  You can fasten them to the wall by putting a long
dowel through the crates and fastening it with a strap of webbing. 
The dowels don't have to extend to the floor if you secure the upper
shelves from moving.