When designers have speced interior glass in archways, I simply used exterior units. The cost of doing a custom single glazed window would be prohibitive. If you're seeking a vintage look, house salvage/vintage historical companies might have what you're looking for.

Your builder may be able to build a custom window for you.

The place I use for mirrors also supplies regular glass and pieces of glass made for direct set into simple, carpenter built frames.

Single glazed exterior casement window will be fine. I doubt anyone makes stock interior-only windows as it is such a rare application.

Don't make it opening and it'll cost you almost nothing.

Find an old single glazed cabinet or window sash and build a frame for it. Try Craigslist / materials / window sash OR glass cabinet doors, etc

As stated above, pick and exterior window or if fixed look at a direct set (tempered plate glass installed on a wood jamb with wood stops.

I definately want it to open

I also think exteriir windows have much thicker frames which will take up wi dow space and not look as good

Exterior windows have thick frames to accomocate operating hardware and weather stripping. Fixed site-built interior windows were common but became unnecessary with the advent of electrical wiring. At that time exterior window frames and sash weight pockets were site fabricated for factory-built sash so there has never been a US market for ready made interior windows.

It appears you're stuck with building a frame for a new or old sash. Or a custom cabinet shop could make the whole thing but I like the old sash best.

I recommend a sill height of 36" for safety..

Your least expensive and cleanest option would be a "block" frame (no fin) aluminum push out (no crank/only one handle) casement window. You can center it in the opening and screw through the frame to anchor so it which wouldn't be seen. I would look for a residential one inch frame for the narrow look.

Not many of those companies around anymore and mostly in the west. Two in my area would be Paramount Aluminum in Oakland and Ventana in San Francisco. I'm sure either one would ship a frame only (no glass) to you if you have no local choices.

If you're willing to pay for an exterior window, look at the Marvin Ultimate Inswing Casement with single pane glass.