Alot of your bigger antique stores have them and alot of times everything you need.Dont know where youre from but Lancaster Pa, Renningers,and black Angus probably have them.

I'm in central Texas and have not had any luck finding these in local antique stores. The occasional windows I find are never the right size. Need to find a source on the web if at all possible so was hoping to find someone manufacturing "replacement" interior transoms.

Any custom millwork shop can make these for you, in the wood of your choice. Find a local shop. They can also make the doors and jambs if you like.
Casey

There is a company in North Carolina that makes some really cool interior transoms. They have just about anything you would ever want. Great source! Go to Transomsdirect.com

Just checked out the company above, and they have a 12x32 for $70. I'd check with a local millwork place for a competitive bid, that way no shipping fees.
By the way, when I was house-hunting, all the interior transoms I saw, and ones in friends' houses, were plain glass for interiors--only exterior transoms had muntins or stained glass. My own transom over the front door is stained, an art nouveau pattern--while that over the back door is plain. The front one matches my sidelight.