<p>I have a concept that's roughly 18"x30" (about 457x762 mm), and I just realized that the Makerbot and similar printers only allow dimensions around the size of a piece of paper. What are other options? Are there large 3D printers I just don't know about? I looked up TMC injection mold, but couldn't find anything on size (most links go to PDFs with printer images). </p>

<p>Is injection even in the same "world" as 3D..? Sorry for confusing the two.</p>

Yes
If you have the money
Either read
here
or look at the ones I plucked out of that list to show you:
First of all, 3D printers on FDM basis can become quite large, if you pay extra. And the price goes up really fast as the dimensions grow because the demands on stability grow exponentially.
Larger Printers?
One example of a "scaling" printer family is the CR-10: The baseline CR-10 is about 300x300x400 mm, so about 12x12x16 inch for 400\$. Not enough? 400x400x400 on the CR-10 S4 for ca 760\$. 500x500x500 on the CR-10 S5 for 900\$. That's 20 inch in all directions. But that's still not enough, isn't it? Well, if you need to go larger, you need to go professional...
Going Big!
Well, there are printers like the gCreate gMax 1.5 XT+ (406x406x533 mm) for a mere 3000\$. That's enough, isn't it? Well, we can even go
bigger
: 610x610x610 mm for a mere 3500\$ on the Modix Big 60.
Still not satisfied? Industrial machines can go even larger! A BigRep Studio for the
little
price of 50000\$ could achieve 500x1000x500 mm.
Even more? Go Parametric!
As amra mentioned, the
Hangpringer
project of printers has no "set" build size, but you will have to provide a build platform of fitting size and a room it can work in undisturbed and with filament supply ready... just... bring a ladder? And be wary of people tripping over wires... and somehow find a ginormous, evenly heated bed somewhere...
Ways around
Assembly
That is a little overkill, isn't it? Well, the first thing you need to remember when 3D printing is: unless you make a functional part that has to have certain dimensions, feel free to scale it down if it is just for visuals. Then, you can assemble prints. Like, take a smaller printer and print the object in halves or quarters. Then glue it back together.
Order
You don't want or can't assemble the part? Well, there are printing services out there that have these larger scale printers I mentioned (or at least similar ones) that offer to make your parts for you and then ship them to you. Usually, that is much more cost effective than buying one of these ginormous printers yourself.