I'm not sure if they do diagnostic work or just procedures, but the Surgery Center of Oklahoma is an interesting cash-only option. I know there are a number of similar operations. Concierge medicine is industry term doctors who directly bill patients for very personal care and skip insurance. Kinda like having a lawyer on retainer.
Many market makers profit off speed and information advantages while providing liquidity. There are alternative market structures like frequent batch auctions that would allow better trading, lower spreads and negate the HFT speed arms race. This paper is a good overview:
https://surgerycenterok.com/pricing/