FWIW, every AAHA-accredited clinic I've every patronized had one. That's only the top 15% or so of all vet hospitals--meaning 85% of all clinics are not AAHA accredited. I can't say whether it's true of all of them, but that experience covers three different states. They tend to be significantly more expensive than surrounding vets generally due to the commitment to extra training and investment in diagnostics -- but I've never had to wait for an ultrasound at one of them. Vet school clinics also always have them. In one case, being able to do it immediately helped avoid exploratory surgery that otherwise would have been recommended. Digital radiographs are also pretty standard at this type of clinic.
Maintaining AAHA accreditation seems to require a significant clinic investment in diagnostic technology and training...and that does drive up prices. So there's a gimme and a gotcha to it. "Country vets" practicing older, spartan facilities can simply charge less, and that's what many clients want -- so most people who don't need ultrasounds don't want to pay higher rates for everything that would be needed to enable the vet to pay off the loan on the expensive new machine.
I think we're entering an age of two tiers of vet practices -- high-end, full-service practices with cutting-edge diagnostics and the latest, greatest tech (and sometimes even specialists under the same roof) vs. bare-bones, low-cost clinics with minimal diagnostics (some I know don't even have xray capability). The middle seems to be eroding as people retire and sell-out their practices to VCA/Blue Pearl/Banfield -- the ones in the middle are sandwiched uncomfortably between low-cost practices taking away bread-and-butter wellness visit clients based on cost, and high-end practices that offer a lot more in beautiful, new facilities. As WM rolls out its low-cost, no-frills wellness clinics in stores, watch this trend accelerate!