Having bought a Roadster within the past month, I'll share my experience.
Extensive searching found only 21 cars for sale in the United States (fewer now). I wanted to spend no more than 65K total (car, tax, transport, registration, etc), which ruled out most 2.5s and Sport models (and a 3.0 at $125K+).
Of the 10-15 left, a couple were on consignment with disinterested agents - "Is your Roadster still available?" "Yes" "Can you tell me about it?" "Not really" "Okay... well I'm interested and am ready to move forward if everything checks out" "Call my main office number and they'll put you in touch with the owner". No thanks. Others were listed but not actually available (Shift.com). Some were iffy - 2.5 bumper on a 1.5 with no back story... was it an upgrade or a "repair"? need to know these things. A Validation Prototype was (and still is) listed, but the seller seemed to know nothing about the car. Claimed it had a 2-speed transmission and 244 mile range, and was very vague when answering questions. Didn't feel right.
Now we're down to 3-5 potential cars. Signature #83 (Very Orange) was my first pick and the seller was very friendly and helpful. Unfortunately, I was a few days too late. Someone else got a great deal. It had only been listed for a few weeks. The car I finally decided on was at a private dealership in a nearby city. They were happy to give me a virtual tour of the car via Facetime and find answers to any questions I had. This car also had only been listed for a few weeks and supposedly received a lot of interest.
As far as the newer models reducing interest in the Roadster... I stood in line for the Model 3 and intend to pick up a Model S for the family when our Volt becomes too small. For me, this is the perfect commuter/fun car. Most people (outside of the EV community) don't even know that Tesla made it. Many I've spoken with thought it was "the new Tesla" and are surprised to learn that it's 8 years old.