Where do I find glass to smith with?

Glass items are made from Refined Malachite which is in turn made by smelting 2 Malachite Ores.

Malachite Ore can be found:

• 7 veins in Steamscorch Mine, directly Northeast of Kynesgrove.
• 2 veins (along with 2 Ebony Ore veins) at the top of Throat of the World.
• 4 at Labyrinthian Thoroughfare (Hjaalmarch)
• 3 at Labyrinthian, Tribune
• 2 centered at 150 ft NE of Kynesgrove
• 1 at The Brinehammer Below Deck (The Pale)

Refined Malachite can be found:

• 3 centered at 180 ft NE of Kynesgrove
• 2 at Swindler's Den (Whiterun Hold)
• 1 at Saarthal Excavation (Winterhold)
• 1 at Understone Keep (Markarth)
• Sold by blacksmiths and general goods merchants starting at level 18 (20?)

"Glass" forged items are actually made from a green metal named Malachite. It's not actually glass. So once you take the perk, you can create items based on Malachite.

I assume you're referring to the Glass Smithing perk in the Smithing tree.

While I haven't taken that perk myself, according to UESPWiki, none of the glass pieces take glass as a raw material. They all tend to take some combination of leather, leather strips, malachite, and moonstone.