You can download it here: https://www.obj-sys.com/products/asn1ve/index.php
We often use screen shots taken from this tool in the developed examples shown here and in the graphics of IEEE and SAE standards that SCSC has edited over the years.
This tool will allow you to hand craft up and create any ASN message you may need. You can also view and edit the messages created by others; checking them for validity against the ASN specification that was used. We often suggest to clients that they purchase a copy of this tool for each developer working at the ASN level of the application. You will also need a valid copy of the ASN specification you wish edit (in this case J2735) from SAE.
Objective Systems is an ASN.1 supplier. They also make a full line of ASN encoder and decoders and have some compiler tools specifically developed for needs of the DSRC community.