I have purchased from stockinteriors.com. I cannot comment on their quality for the Grand Prix platform, but I can say I did it my old Avenger. It comes pre molded, no holes for seats, seat belts, console...... anything etc. Best bet is to un roll it, leave it in the sun to flatten out. Pull out the old carpet, set the old carpet in the new carpet and cut the new one leaving at LEAST a good inch OVER lapping while using the old carpet as a big stencil. MAKE SURE you take any clips, retainers etc off the old carpet and staple it, glue it whatever onto the new carpet. I failed to do this in my Avenger and was constantly having to tuck it back in under the molding (then again I did NOT leave an extra inch, causing it to be short). Also I would highly recommend getting some jute padding for carpeting and cut it in the shape of the floor pans and spray glue in or just set it in. These aftermarket auto carpets from ANY mfg are not perfect so expect them to be a little off. So by raising the height of the carpet using jute padding it will help the wrinkling, folding, waviness of the aftermarket's mold. Also the jute padding will help with sound deadening.
Hope I gave a little insight amidst my rambling.