Rebuild Indexes on SQL Server Standard edition
Depending on your version of SQL Server, you can do index rebuilds as an ONLINE operation. This is an Enterprise feature.

Failing that (ie Standard Edition etc), look at re-organizing indexes - these will keep them online. Microsoft best practice says that you should generally not do a rebuild operation unless fragmentation is over 30%. You should always do some form of intelligent rebuilding - i.e check fragmentation and take an appropriate action. I can't recommend Ola Hallengren's maintenance solution enough for dealing with this.

https://ola.hallengren.com/

If you are worried about the blocking effect of index rebuilds, I would recommend this sort of approach.