There are two possibilities here
- You want HTTPS because you want to secure your data transmission
- You want HTTPS only to show SSL lock. So that mobile apps can connect smoothly.
If you come under #1 - I suggest you to buy proper certificate from any vendor and install it at tomcat level so that all your microservices can use it. I took a free certificate from letsencrypt for 3 months and did the same. However, installation can be real painful as you have to convert those certificates to JVM supported format etc.
If you come under #2 - I suggest you to use reverse proxy service like cloudflare. It is very easy to configure. Also, cloudflare issued SSL will be free.
i.e. Your traffic from client to cloudflare will be secured and have SSL lock. However, traffic from cloudflare to your server will be HTTP.