I've used this underwear for a couple of years now and like it. It is well made, warm, fits me well and dries quickly.
http://www.hanksclothing.com/codet_quilted_thermal_underwear.html

Quilted fiberfill underwear used to be easy to find and was readily available at most sporting goods stores, including the sporting goods section of K-Mart. Not so anymore. The link above is the only place I have been able to find it.

I know Codet for their wool pants and other wool outerwear. Here is some contact information for Codet / Big Bill
You might also try Carhartt clothing. My Carhartt winter work coat has a quilted removable liner.
Hopefully this is some use to you, although I haven't answered your question.

I'm wondering if this stuff might be more useable as a mid-layer rather than base layer. I like baselayers that wick moisture, and mid-layers that insulate. YMMV, of course!

Unfortunately the site doesn't say anything about the fabric or insulation or how much this stuff weighs. Considering how much we have to pay for Montbell Thermawrap, this might be a great substitute, if it isn't too heavy and doesn't contain cotton.

I use the top as a mid layer and use the bottoms as a base layer.

The ones I have are men's large. The top weighs 13.4 ounces and the bottoms weigh 11.6 ounces.

Outside and inside fabric is nylon which I would guess is about 2 ounces per square yard. I wish it was 1 ounce to save weight. On the plus side, both garments are sturdy enough to be worn as an only garment with nothing over them.

Top has two handwarmer pockets. Zipper is YKK ladder style. Top has a flap in the back that extends downward to reduce, I'm guessing, wind from blowing in the backside. Snaps allow one to fold it up out of the way if desired. The top looks like a jacket when worn on the outside.

For both garments insulation is some type of fiberfill (polyester). Ribbed cuffs and collar are nylon or polyester (I can't tell). No cotton.

The insulation in these garments is thin.....maybe 1/8 inch thick or so. My comparably sized military surplus jacket liner top, for example, weighs a couple of ounces less and is about twice as thick. The jacket liner has very thin nylon, however, and is fragile. I always wear something over it and it tears at the seams now and then.

Thanks for the specs on weight and materials! Several ounces heavier than Montbell Thermawrap, but they really do sound like a viable alternative for those whose budget will not stretch to the mega$$$ Montbell stuff!