Please tell us which version of Microsoft Office you're using. (The pre-installed templates vary greatly between different editions).
My own instinct though would be to design my own blank invoice in Word and then use Save As > Template so that it was readily available whenever I needed in it. (Indeed, that's exactly how my own business invoices are produced).
You should never have any problems (with adware, etc) when downloading Office templates,
as long as you get them from Office.com (which is Microsoft's own repository of free resources):
https://templates.office.com/en-gb/Invoices