JSR-275 has been rejected, the Units of Measurement API for Java project is a set of interfaces, but haven't found an open source implementation.
On this post: Which jsr-275 units implementation should be used? the project owner mentions the implementation was going to be ready by the end of last year on JScience, but didn't find anything there to convert between weight or length units and when I looked for JScience on https://maven.java.net/, I found it, but the JAR wasn't even in the directory https://maven.java.net/content/repositories/snapshots/org/jscience/jscience/5.0-SNAPSHOT/, so I had to get it from somewhere else.
Has this project been left behind? And is there currently an implementation for conversion of Units of measurement in Java and even perhaps a Maven repo?
As far as I know, JScience is very much alive. The project is currently being migrated to Java.net, and the migration is not complete. That is most likely the reason why you are unable to see snapshot JARs for 5.0. In fact, the most recent snapshot was prepared only after resolution of a particular configuration problem reported in the Java.net JIRA. May be you ought to wait a few days, or probably send a mail to the project administrator on what JScience POM must be used in the interim.