There'* also carsoup.com, cars.com, autobytel.com, and thriftynickel.com (search by city, looks like Topeka, KS is the closest to you). And check your locals as mentioned, many if not all are publishing classifieds online now... often you can also set up search keywords to e-mail you whenever matching ads are placed (like an ebay 'Favorites' search).
Autotrader worked out best for me... found both my '98 Bonny and my '99 Aurora there. If you're looking for a specific model, you've gotta recheck the listings ever 1-2 days as they are constantly changing! GL -
Current GMs: '95 Chevy Suburban C2500, '69 Pontiac Executive
Former GMs: '83 Buick Park Ave, '73 Buick Electra, '92 Chevy Caprice 9C1, '98 Pontiac Bonneville SLE, '99 Olds Aurora, '00 Buick LeSabre