There is a robust second hand market for ultrasound systems and probes, just google for those companies or search on eBay.
There are also research platforms out there for imaging from various universities, some of which are open source. They aren't nearly as good as the commercial codes, but a few grad students can't compete against teams of full time professionals working for decades. Commercial codes are unlikely to be open sourced. Many are tied to the architecture of the systems, and most are reused in later systems and so are still useful decades later. It's highly unlikely that any company would open source them, it's just not how they operate.
As for explosions of technology and advances - yes, I do expect more advanced electronics and signal processing to have a large impact on ultrasound in the coming years, but it is more complex than most in this thread give credit for, and will take time to appear.