varies by state
In WI, senior PRs without medicare are more likely than not denied health insurance. On the other hand in MN, permanent residents over 65 denied health insurance can buy a state mandated plan for high risk pool where the premium is some 10% higher than the market rate.
An important point to note is that PRs after 5 years are eligible to buy Medicare. In some states like MA, the state pays the Medicare premiums through the Medicaid system. As per federal law, only US citizens can get SSI. Those who get SSI are eligible to get medicaid. So technically, it will be hard to get medicaid until one is a citizen unless they live in states that still allow medicaid for green card holders. Prior to 1995 (?) new PRs could go straight for Medicaid and also SSI. Now sponsors are expected to pay until 10 years or until the sponsored person obtains US Citizenship.