Amazon has released SFTP services for S3, but they only do SFTP (not FTP or FTPES) and they can be cost prohibitive depending on your circumstances.
I'm the Founder of DocEvent.io, and we provide FTP/S Gateways for your S3 bucket without having to spin up servers or worry about infrastructure.
There are also other companies that provide a standalone FTP server that you pay by the month that can connect to an S3 bucket through the software configuration, for example brickftp.com.
Lastly there are also some AWS Marketplace apps that can help, here is a search link. Many of these spin up instances in your own infrastructure - this means you'll have to manage and upgrade the instances yourself which can be difficult to maintain and configure over time.