What you need are carbon steel double edge blades, not stainless. The stainless blades are softer and not suitable for our use. Carbon steel blades are usually black or blue. Google "carbon steel double edge razor blades" and you will find sources. The two planes in your post, the Little Giant and the Zip have a center locating rib in the blade bed. The rib may not allow you to retract some brands of blades for a fine cut, so try to buy a few to test before you buy a pack of 100.
Norm Furutani- lifetime glider chucker