The left bolster on the seat of the driver seat on my 1998 Deville is broken down. I bought the car used and it looked like the pervious driver was either a large person or just sat on the left side of the driver's seat.

Is there a simple way for my to repair it? How would I do it and what sort of cost is involved? Do I need an upolstery person?

LOL I have the SAME EXACT problem on my 96, it feels like im gonna fall...well sorta. Its not that noticable, I only noticed it after my back started to hurt after long drives. ANy help is appreciated!!!

I put over 300k miles on my Mustang and in the course of that use my seat sagged pretty good. I took it to the most reputable auto apholsterer in my town and they did a great job of building the seat back up. Didn't cost me much either.
If you want to try it yourself I'm sure you could remove the cover and build the cushion back up with foam rubber, you would have to shape it to match the other side.

If you're gonna try and do it yourself. go for the firmest foam you can get and you might as well do both bolsters while you're at it - nothing worse than having one bum-tock out of whack or on a lower plane than the other - enough to ruin your whole day,,,,,,