Glass Slopes were introduced with Patch R36 on November 15th 2016.
2 of these Slopes can be made in a Processor from one block of Glass each. No crafting recipe has to be unlocked for that.
Glass is made in a Forge (together with Fuel) from Sand that can be found on many Shorelines, in Dunes, all over the floor of Oceans and on sandy Islands as well as around/under some lakes like in Savannahs or oases of Canyons.
Since R43 in May 2017, inner and outer corners blocks can be created for Glass Slopes too by putting them into a Processor. No crafting recipes are required for this.