I have an 1993 Featherlite w/the smaller windows like this (hopefully the picture will post - if not 5th item down on this page: http://www.horsetraileraccessorystore.com/window_feed_doors.htm)

Drop-Down Windows serve as "feed-door" and window combined! Top-latch handle opens from outside, hinged at bottom.  "T-slider" style lower glass panel allows screened ventilation with window in upright-closed position.

I've done several searches and can't find a source for them.  Anyone have some handy links or maybe a better search term than what I have been using?

I would contact Featherlite first, see if they could help, give them the serial number, they should be able to "match something"... Here's some other drop down windows...

http://www.statewidealum.com/livestock-trailers-statewide-alum.htm

http://www.hehr-international.com/0280feederwindow.htm

Thanks Retento -

I was hoping to not have to replace the whole window but just find something that could be added on. 

I'll contact featherlite but do you think that there might be a way to just add on the bars to the inside of the window?

A few years ago we purchased a used trailer that had window mounted bars. One window's bars were badly bent, a second was slightly damaged. I took them both to a welder, who easily fixed the one that was less damaged. He determined that it would be easier to fabricate a new one, than rebuild the bent and broken assembly. I picked up some materials, cut them to length and dropped them off. In less than 1/2 an hour he had the new one built. Total cost for his labour was $35 for both.

If you know a welder, he should be able to custom make some bars for your windows. The fixed ones are much easier to fabricate than the drop down type. You might also consider using stainless materials instead of aluminum.

Gard

I had a 96 featherlite that had the same thing and I purchased these.

http://www.nrsworld.com/istar.asp?a=6&id=HA-600!PRCHO

Cheap and easy worked good.  Horse got air and couldn't stick its head out to get taken off by truck, or get a bug in the eye. 

We put screens in one of our trailers two years ago. They do hold up and work well. Be sure to get the zippered type.

Gard

My brother's trailer came with those Professionals Choice ones in it. They have elastic loops in the corners and they stretch out really easily. We opted to replace them with this style of Professionals Choice ones with the velcro closures: http://shop.hawthornecountrystore.com/browse.cfm/shop-by-brand/professionals-choice/professionals-choice-trailer-window-screen/4,3024.html?AFF=BASE

We've had them in all summer and hauled almost every other day, they're still going strong. Our horses only lightly touch them with their noses, but haven't caused any damage. They're also easy to undo with the velcro closures.