Single edge razor blades
Single edge razors and blades have been in existence since the concept of shaving started out a few thousand years ago, they are still produced all over the world and getting some for your vintage or modern single edge razor is not a problem if buying online is within your means. They are normally a bit thicker than double edged blades, and should keep an edge for longer, but like any blade choice it is not only longevity that plays a role in that decision, but also comfort, sharpness, branding, presentation and packaging.