I agree with all of the above; you need a wide lens. Cropped sensor, minimum 10mm, full frame, minimum 17mm. Although you want to be a debt free business, if you want to pursue this seriously, get the widest high quality lens that you can and the fastest. It will pay for itself quickly. I had my Canon 14mm 2.8 stolen and am working with a 17-40. What I wouldn't give for that extra 3mm right now!
While there are ways to stitch together multiple images to get a wide angle view, the result will ultimately be unsatisfying. Even if you can pan the camera around the nodal point of the lens instead of it's tripod mount, the perspective will change with each shot, and the floor and ceiling lines will curve. Buy or rent a wide-angle or a wide zoom. For your crop-frame sensor, something like the Canon EF-S 10-22mm will do it. There are lenses in the same approximate range by Sigma, Tamron and Tokina that will save you some money, some rentable at Borrowlenses.com for as little as $20 for 3 days. Use a tripod, and the slow lenses should work out fine for you. You'll probably want to get to 17mm or so in a full-frame equivalent to make sure you're covered.
The question nobody asked is, what lenses have you got?
You could take multiple photos and stitch them together in Photoshop. Make sure to use a tripod. I personally would borrow or rent a lens.
Thats a rather strange question, how about renting?
You are in the USA, so the possibilities are endless, for example:
http://www.lensrentals.com
and
http://www.borrowlenses.com
I have a couple of local agents that want me to shoot some photos of upcoming listings. I currently do not have a wide angle lens because I am running my business debt free and don't have enough to buy one yet. What are my options? Thanks!