Where can I get replacement brake lines for a 2002 Buick LeSabre?

An Auto parts store will have the lines you need. You can by line sections from 1 foot to 6 feet long and do it the way or you can buy a roll of line in the size you need but you would have to flare the ends yourself to fit your particular needs. A store like Autozone can rent you a flaring tool and sell you the fittings you need or you can remove and re-use the old fittings.