You can find DaVinci syrups (and Torani, which is just as good, IMHO) online, but also at discount places like Marshall's, TJ Maxx, etc. The easiest place to get them, if either of those options is not the best for you, is at a local coffeeshop. The type that make lattes--and privately owned. Ask the owner if you can order the syrups through them. Several people on our board have done this, as have I, and though it can be expensive, it's cheaper than the shipping and handling of buying it online. They didn't have any at my local TJ Maxx.
They really are great, though!
As to Stevia, no research has been done on it here because it's not technically a legal sweetener. It's legal as a supplement (like vitamins). A lot of people blame this on the corporations that market other sweeteners and on the sugar lobby. However, Stevia has apparently been used in other countries safely for many years. It's really a personal choice.
One website I read said that she found the best results when she mixed sweeteners...less aftertaste overall.