
I just ordered this double niche from http://www.tileredi.com/, and had an email within minutes saying it would be received in 3 to 7 days. My local tile shop had similar products, but the sizing is what I like about this one. 16" X 20", so it should fit between existing studs and minimize any framing I need to do. I'll verify that for you once it arrives. Just screw to studs, install underlayment, silicone the seam between the niche and underlayment, and tile over.
UPDATE: Ordered at 930 this morning, email around 3 this afternoon from UPS letting me know it had shipped. We like fast.