Transparent items like glass

Solution 1:

Leaves are slightly transparent and can be harvested with shears. Unlike "natural" leaves, harvested and replaced leaves will not decay in the absence of logs.

Fences are probably the cheapest alternative, you can stack them as much as you like in recent versions. Nether Brick Fences are harder to obtain, but might go better with your decor :-)

Iron Bars would also work; they function similarly to glass panes.

If you're in a snow biome and patient, Ice would work. Lay down a single layer of water source blocks, expose it to air (vertically), wait for it to freeze and repeat. Be sure not to put torches near it!

That said, I might make a little trip and try to stock up on a ton of sand (4 stacks or so), if you convert it to glass panes you can break and replace them (unlike blocks) so even if you remodel you won't be losing anything.

Solution 2:

Water is a great alternative. It's easy to find and replicate (it's one of the cheapest resources in the game!), and provides an effective barrier to mobs, as they will automatically 'swim' upwards in water. This means (with a bit of careful engineering) you can actually use your windows as mob traps! It's almost completely transparent unlike other block-based alternatives, and provides a relaxing blue tint to your view :-)

Alternatively, for some nice big picture windows that are mob-safe, a Nether Portal will serve you well.