The reason why you shouldn't perform surgery on an insulated mitten is that an insulated Gore-tex mitten is made completely differently than a Gore-tex shell mitten. the insulated ones are built using a mitten-shaped membrane that is inserted between the outer shell and the insulation - this means they don't have to tape the seams to gt it waterproof. As hell mitten, on the other hand, is taped. so if you cut the lining out of the insulated mitten you have the fragile membrane exposed and it won't wear well or work well.
REI has these: http://www.rei.com/product/787289
I have some outdoor research shells that I like, they came with a removeable glove that I don't use with the shells but sometimes use on its own. They might be the saem as the Latitude Mitts that they currently sell, although those seem a lot more expensive than mine were just a few years ago.
Making your own is pretty easy , but sealing the seams on your own is not. I used homemade shells for years, until the one trip where I got soaked due to the non-sealed seams. I went right out and got the OR shells and I'm glad I did.