Finding the right plumbing company is really a matter of doing some research. A few quick searches on the Internet should help you find out which companies are the highest rated in your location. Based on this, you can then call around for quotes.
One thing I have learned over the years is that sometimes it's not best to go with the lowest quote. The expression "you get what you pay for" is really true in most cases. Sometimes it is worth paying a little bit more to know that the job is done right.
Once you find a plumber you want to work with, be sure to get a contract in writing before they begin working on fixing the problem. That way you won't wind up with a lot of extra charges on your bill that you didn't agree to.