Buy the actual printed art. https://www.wallartprints.com/architectural-photography/ That wayfrom the photographer is ensured to get paid for their efforts and quality control is in their hands for printing.

Or search on https://www.foap.com/photographer where photographers are selling their photos.

Also gettyimages.com or shutterstock.com may have images that can be purchased to get the resolution you need to print at scale.

There is art for public domain at New York Public Library. They have a really nice collection and I love to browse the site. https://www.nypl.org/research/collections/digital-collections/public-domain.