Step 1: What data does your organization already have? Or other local organizations that are similar to yours?
I would closely examine their annual reports to see what data they're collecting.
If for some reason you can't get a hold of their annual reports on their website or from your client, you could try looking for their Form 990 (official tax document they have to file with the IRS if they want tax exempt status) on the Foundation Center's website. The Form 990s at least give you a sense of their finances.
Step 2: Look for local statistics.
Below are suggestions for getting a sense of Spokane by the numbers.
Step 3: Look for local news stories.
What press has your organization already received? Other local organizations that are similar? As well as what are the news outlets saying about the issues your organization is trying to solve.
As you are finding information...
Be sure to carefully evaluate the authority of the organization offering the statistics, and if there is any information about how the statistics were collected.
Keep an Eye Out as You Are Researching...
As you are finding background information or articles on your topic, take note of any relevant statistics. Some scholarly journal articles may have done detailed studies on the issue and generated statistics. Or a news article may mention a juicy statistic to jazz up the article.
Note the source -- you may have to dig into the references if it's a scholarly source, or look for who was interviewed in the news article.