Retirement Savings options for Non 401K employees

Our current employer does not offer 401K. Started to invest on Roth IRA from past 5 years. The max is only 5.5K yearly contributions we can as per IRS limits. But some of my friends who work has 401K offered by their employer and they can contribute max of 18.5K year

to their 401K yearly. I wanted to contribute more towards retirement. Apart from this Roth IRA, where else can I save or contribute for my retirements. Are there other 401K plans which individuals like me can open on our own and contribute .

Thanks in advance.