As Truman said Advertising Agencies.
Also to add.
Start small with a local ad agency.
Sometimes a small agency might be one guy.
Be Prepared before you meet with him.
Have some samples of 15 and 30 second clips.
If you don't have any than pick a couple local companies and just simply pretend that you had to write them a jingle.
Or if you contact the agent they might have a few low budget clients that may want a jingle but just don't have the high ad budget that a large company might have.
That's where you get your start.
I wrote one for a local audio video store that has been playing on local TV for about three years now... To bad I don't get paid every time it's played..
Made a Whopping $400 off it.
It was fun and a good learning experience.

It must be catchy, Heavy Chorus, and define the Company image..

If you really want to learn that side of the music business. A guy name Jefferey Fisher writes alot of good books.
He also has a newsletter call money making tips in music.
http://www.jeffreypfisher.addr.com/moneymusic.html

ANother Note..
Large Ad agencies that work with large Companies that have large budgets only use Big Jingle Houses with Proven Track Records.
It's just as hard to write a Jingle For McDonald's as it is to Get Signed to a Major Record Label.. Not kidding...

There is a saying in the Advertising Business.

"The only way to get in the Ad Business is to be in the Ad Business"

Even our Local TV stations use Nashville Jingle Houses. How that works is that TV stations that sell Advertising space for Local tv time have package deals that include jingles. They wrap it in so that they can make some money off it too. twice a year the jingle house REP comes to town and gathers up all the work.. A local guy doesn't stand a chance when you have only one or two TV stations in a small town. Same with Radio Stations...
So the bottom line is that you can only go after low budget clients that don't think they can afford a jingle.
and find a local Ad Agent willing to take a chance on you.

David