a. Avery offers Microsoft Word templates on their web site. You must provide name and email address to get to them. You could download a Microsoft Word template, open it in LibreOffice Writer, and save it as a LibreOffice template.
b. Install gLabels (http://glabels.sourceforge.net/
) from the Mint repositorties. It doen't appear to have a correct template as part of the package, but you can create custom templates. The web site has excelllent documentation. gLabels lets you design and print labels, business cards, etc, of nearly endless variety.