I am so happy to hear that your private teacher caught it and is recommending a *smaller* instrument for you. It’s so deeply ingrained into the viola tradition that “bigger is always better” to get the rich viola tone. I am 5′ 7″ with smaller hands and the 16″ viola I played on for YEARS turned out to be the wrong size for me and caused a couple lasting forearm/wrist issues that continue to this day. I now play a 15″ viola for comfort-sake. While I could probably give a 15.5″ instrument a try, I’m just not interested. 🙂
As for places to find replacement instruments, don’t rule out Superior Violins (www.superiorviolins.com) which is run by Michael. He has a few decent violas as well and is super reasonable with discussing rental terms. I’ve personally played two of the instruments he has available – both in the 15″ variety. They’re quite good. Not master level, but an excellent price-to-quality balance.
How long have you been playing? What level instrument are you looking for?