The reason you can't find them is because there are only a few companies that manufacture them, mostly vinyl. They were very much a thing of the 80's and 90's. A lot of window brands use the same garden window manufacturer. I've heard of RLang for a long while but don't do business with them since mostly glass shops deal with them. I wouldn't be concerned about the paint vs anodized.

@fenestration_Taylor: thanks! Is there a reason people don't do them now? They seem like they would add so much space and some light above a kitchen sink?

R Lang is a very well known company in the Bay Area and for years was just across the bay in Richmond. They moved their manufacturing to the central valley and make both an aluminum and vinyl garden window as well as many screen products.
Garden windows have fallen out of favor over the last ten years and there isn't nearly the call for them that there once was. I think it sounds like it would be perfect for you, however, as it would add both light and space. If it is still dark in your kitchen after the addition, think about a skylight or a Solatube unit. Solatube would be less expensive to install.

Garden windows have always been notorious leakers, regardless of material.
An aluminum unit may as well be used as a refrigerator when its cold out, because being exposed to the elements on 5 of 6 sides combined with a highly conductive material is not a good combo. In a very moderate climate I suppose that is mitigated, however it does explain why there is very limited availability, as the availability corresponds to saleability on a larger scale.

HomeSealed is correct, they are very problematic, no matter the brand or material. I never recommend them.

I don't know if it's true, but I've been told that the reason they're so expensive is because the manufacturer anticipates warranty claims and therefore charges enough to replace the entire unit under warranty in the original price.

Hmmm, that is interesting - thanks everyone. Lots to think about. Now I am not sure what to do!

Why not look at a quality vinyl product and inquire about a box bay window?

Here is a link that might be useful: Box Bay

+1 to MWM's suggestion. Won't have quite as much light as the GW, but save you a lot of heartache down the road.

I installed two large, 6'x4' aluminum frame sun garden windows in my third floor apartment in Cambridge, MA back in 82, replacing a triple window setup. They were simple aluminum frame windows with adjustable wire shelves and an awning top that screwed directly onto the framing from the outside. I was there for 30 years and they never leaked or had any problems at all, and as far as I know, they are still there. They did get cold in the winter.
Now I am looking for something similar to replace several jalousie windows in an unheated sunroom. I am just beginning the search, but the closest I have found so far is a company in PA, SolarInnovations that does custom aluminum or vinyl windows of all kinds.