I have one from Prince Lionheart that I got at BRUS 6 yrs. ago for ~$20. It has about 200,000 air miles on it at this point. It did get chomped by some bag-sorting machinery in England 2 yrs ago, and the airline gave me a generous check to replace it. However, the only part that was torn was the thick vinyl on the bottom, so I got a vinyl repair kit from an auto-parts store for ~$4; it only took about 15 minutes to repair it, and you can't even see where the damage was.
I saw some Eddie Bauer branded carseat satchels at my local Target the other night; it was the first time I had seen them there, but they were on the baby-stuff clearance shelves; I suppose there wasn't enough demand for them in my neighborhood.
Satchels are wonderful; the backpack straps save you when you need to take them to the gate, and they serve as a great "extra" packing space if you are checking the seat. However, if you put loose items in with the seat as checked luggage, put them in a plastic bag, so that they won't give you grief when you need to remove the seat at your destination.