Full disclosure: I work for a GREAT sewer contractor (that's really the business category you want to search for, rather than plumbers. While some plumbers do underground work, as well as inside pipes, you'll be better off finding a company that specializes in sewers and underground work). We're Aces Four Construction, and we give free estimates. But in case you want to shop around, I'm now going to inundate you with all kinds of great tips on finding contractors in general.

Personally, I would stay away from any of the companies with the HUGE ads on the sides of buses or the back of the yellowpages books as they have to PAY for those, so their overhead is going to be WAY higher than a small family-owned business. As you're dealing with stuff underground (and there's the panic/icky factor of sewer issues) people often are at the mercy of their contractor and may be pressured into making a snap decision. Additionally, there are definitely some pirates in this industry, who tell folks they need more work than they need, and that just infuriates me. Take a few deep breaths and do a bit of research first. It'll pay off.

Check the BBB right away. You can check out specific companies here http://alaskaoregonwesternwashington.bbb.org/Find-Business-Reviews/
And they've got a directory of accredited businesses you can search by category. Here is the link.
http://www.bbb.org/western-washington/accredited-business-directory?letter=S

Do you have Angie's List? I don't, but I have friends who have LOVED using it to find their roofers and flooring contractors. The bonus, unlike with the online yellowpages, is that the reviews are real and verified (rather than allowing a company to post its own reviews, ya know?) I'd also check the main online yellowpages for reviews. Superpages.com and yellowpages.com and dexknows.com But know that these reviews aren't necessarily as reliable/screened as those on Angie's List.

Now, sit down, because the costs are going to be in the thousands. Are you in Seattle? Because it's the street restoration costs that are going to kill you. The type of street (both the material of the street, and arterial/side street, bus lane or not) will determine how much, but that portion alone can be between $3k and $9k. And if it's deep, then the thousands of dollars start to add up. Luckily, there are a few "trenchless" options these days that can help avoid the street restoration charges (liners, pipe bursting, etc). They won't always work for your situation, but it's worth asking, and reputable companies should be bringing up these options themselves.

Are you currently without service or was someone able to clear your line? If you're at least partially flowing, HOORAY! Please note that the occasional toilet flush should be alright. But another back-up may be triggered by anything that sends a LARGE amount of grey water down the line all at once (the Dishwasher, Washing Machine, and long showers or baths) so that's really where you should avoid until the repair is done. If you're

Lastly, have you had a video done of your broken sewer and do you have a copy of that video/DVD? If yes, that's great, because you can show it to the contractors giving you estimates, and then everyone is comparing apples to apples. A video will also make for a "tighter" estimate range, as there's a good idea of conditions. If not, I'd encourage you to call someone (it doesn't have to be us, but I'm generally the one who answers the phone, and I'm nice!!) to have a video done. Make sure the company you hire for the video inspectio will GIVE YOU A COPY of the inspection. A few of these companies don't, making it harder for you to get a second opinion or additional estimates, and that is SLIMY!

And now, some general HIRING A CONTRACTOR advice:
1. Always check the BBB.
2. You should also check with the state lni.wa.gov and use the "look up a contractor" button to verify that the contractor is licensed and insured, etc.
3. Get estimates from 3 companies. Most reputable contractors should be able to give you a FREE estimate. I'd be wary of anyone who wants to charge for this service.
4. Read the estimates carefully...often, underground repairs are done "time and material," meaning you are billed for the hours and the parts used, because noone can predict exactly what it'll be like once they start excavation. This can actually be a good thing, because if it takes them less time, it'll cost less money, whereas a flat repair price will need to be higher so the contractor can protect themselves if something unexpected occurs. But I've seen some estimates from companies that LOOK like they are promising you a flat price, but it's actually a variable, so you're invoice will be higher than you expected.
5.Ask for references. Again, reputable contractors should be happy to provide you the names and contact info of others who have been happy with their services.

To recap: Take a deep breath. Don't let anyone pressure you/push you around. Do some research online. Ask for references. Get a few estimates. And feel free to call me at Aces Four Construction to get a free estimate.