How can I have a large 3D printed object? 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). 

Is injection even in the same "world" as 3D..? Sorry for confusing the two.
As Trish states, a print service would appear to be your best bet. Building a printer large enough may work out costing more than the print service, especially if it is only for one print of a proof of concept.

Anecdotally, I visited a 3D Printing shop in Bangkok, JWH High tech Garden, that had a huge Delta printer that had, reputably, cost a million baht (~£20000) if my memory serves me correctly, and was capable of producing prints up to 1000 mm x 600 mm (39.37" x 23.62"):



However, the print service was not cheap at all. In May 2017, I was quoted 68000-100000 baht (~£1360-2000) for some printed parts (the proteins) for a Wilson II 3D printer, which are about only £70 on eBay!



However, it is worth taking into consideration that, at the moment, Thailand is very expensive for 3D printing in general, as it is relatively new there, and so prices for both printers and print services are high.