The Universal Business Language V2.1 OASIS Standard, approved 04 November 2013, is available for free download and free unlimited use here:
HTML: http://docs.oasis-open.org/ubl/os-UBL-2.1/UBL-2.1.html
PDF: http://docs.oasis-open.org/ubl/os-UBL-2.1/UBL-2.1.pdf
XML: http://docs.oasis-open.org/ubl/os-UBL-2.1/UBL-2.1.xml
ZIP: http://docs.oasis-open.org/ubl/os-UBL-2.1/UBL-2.1.zip
The ISO/IEC 19845:2015 International Standard, published 12 December 2015, is the 2013 UBL v2.1 verbatim document with an ISO/IEC cover page, and is available for purchase at: http://www.iso.org/iso/catalogue_detail.htm?csnumber=66370