Standard or guidance for entropy collection
As far as I can tell, NIST has only one official document about entropy collection. SP-800-90B.


  The purpose of NIST Special Publication (SP) 800-90B is to 
  specify the design and testing requirements for entropy sources that can be validated as approved entropy sources by NIST‘s CAVP and CMVP.


It essentially defines a bunch of statistical tests to check the strength of a particular entropy source. Seeking entropy methods that meets the requirements is a good first step especially if you need "official" certification by NIST.