I'm searching, and you're right... in terms of clear glass baking dishes, I'm seeing an 8 x 8 inch, and 9 x 13, but not a 9 x 9. If it doesn't have to be clear glass, there is a Corning 9" baking dish, white with green trim, and the corners look pretty rounded.
https://www.walmart.com/ip/CorningWare-Etch-9-Baking-Dish/16647996
Bed, Bath & Beyond has a 9" white ceramic one by Wilton.
If you do hit up thrift stores, do look carefully for an imprint that says it's oven safe. I simply presumed all square glass pans were Pyrex, and many years ago ended up using a roommate's glass pan mistaking it for the Pyrex one, and it shattered in the oven.
Depending on the recipe, you should be able to use an 8 x 8 pan and just have a slightly thicker (higher) product, so you'd need to adjust your baking times a little.
This is what Joyofbaking.com has in terms of square pans and volume in cups/liters
8 x 8 x 1 1/2 in. - 6 cups (20 x 20 x 4 cm - 1.4 liters)
8 x 8 x 2 inches - 8 cups (20 x 20 x 5 cm - 1.9 liters)
9 x 9 x 1 1/2 in. - 8 cups (23 x 23 x 4 cm - 1.9 liters)
9 x 9 x 2 inches - 10 cups (23 x 23 x 5 cm - 2.4 liters)
10 x 10 x 2 inches - 12 cups (25 x 25 x 5 cm - 2.8 liters)
Read more:
http://www.joyofbaking.com/PanSizes.html#ixzz4rrn4OGue