Standard double-edge (DE) blades fit every safety razor and cost pennies each.
Sharpness and smoothness vary by brand and by face, so a sampler pack is the cheap way to find yours.
Razor blades
Astra Superior Platinum double-edge blades
The most common recommendation for a first blade: sharp, smooth, and consistent, at a few cents a blade in a 100 pack.
100 blades
Buy on AmazonWhat we ruled out
Proprietary cartridges (e.g. 5-blade systems). Each cartridge costs many times a DE blade and locks you to one handle. Extra blades mean more passes over the skin, which matters if you get razor bumps.
Also good
Blade sampler pack. Blades suit faces differently. A sampler costs a few dollars and settles it.