A desktop publishing application from Microsoft that focuses on page layout and graphic design.
I run Pub 2003 on a Win 7 machine...along with several other versions. I am old as dirt too.
I found one source for Pub 2003:
https://download.cnet.com/microsoft-publisher-2003/3000-6675_4-10967173.html?ex=BER-1278.1
I can't guarantee the file, but CNN is a reliable source. I think the file is for Office 2003 so if your key code is for the standalone version of Publisher it won't work. If your key is for Office 2003 then you can do a custom install and install just Publisher.
I can't find it right now, but there is a MSFT site where you can download old legacy versions of Office...you might find Publisher or Office there.
Google 'Publisher 2003 download' or Office 2003 download...be careful of the referenced links.
LibreOffice Draw will open Publisher files. I don't know how much editing you can do, but at least you could access the content.
Before you buy another copy of Publisher know that you may not be able to open a Pub 2003 files with the newest versions of Publisher, plus MSFT has announced they are dumping Publisher in a couple years. Check out Scribus or perhaps the Serif / Affinity Publisher. There is a trial copy and it runs on PCs and Macs...looks like it is half priced right now: https://affinity.serif.com/en-us/publisher/?utm_source=google&utm_medium=Paid+Search&utm_campaign=18244115922-EVERYTHING50JUNE&utm_content=154909585097&utm_term=serif%20publisher&gad_source=1&gclid=Cj0KCQjwpZWzBhC0ARIsACvjWRPQ7jZfsZGoKdJVm2V9s3Tg3GcSOWgaws7CRjlFd1ZwNqDVKD7W9u4aAsz5EALw_wcB
DavidF