Hi guys. I rarely visit here but do drop in from time to time,, so forgive me bringing up an old post, but I thought Id offer my opinions.
I have and still do auto upholstery profesionally for 10+ years. The ONLY carpet supplier we use is ACC. You can find them on the web at http://accmats.com/
They are in my opinion the leading aftermarket supplier. Our Bonnevilles use a Mass back carpet. That basically means that the backing of the carpet is a thick rubber type backing. A cheaper backing is available and will work just as well. The carpet in our cars have plastic strips along the edge that snap into the body that help hold the carpet in place. I know of NO aftermarket supplier that places these on their carpets. All aftermarket carpets will require some trimming to fit, but as I previously stated, I prefer ACC.
To find them you may be able to visit your local auto upholstery shop and ask them if they sell ACC. Hope this helps.